Mastodon: the Obligatory Explainer

Mastodon has been experiencing a huge spike of growth in the last few weeks; as of this writing, over 330,000 users; up from around 20,000 at the end of last year. It’s also getting a decent amount of media attention, including a great piece by Sarah Jeong (full disclosure: Motherboard is operated by Vice and so is Waypoint, a website I have written for) and some not-so-great hot takes from tech journalists.

Mastodon has a user guide buried in its documentation, but it’s written more to be comprehensive than comprehensible; it also has a small explanation for new users which is very good, but by necessity short, and it doesn’t help people trying to understand the whole “instance” thing before they join. This is an attempt at putting together a shorter guide that explains some of the salient things that confuse people about the platform, and puts things into context.

I just want to try this thing, I’ll come back for the details later

If you’d like to just jump into Mastodon and check it out: Mastodon has different servers you can choose from;,, and are three good, large ones. There you can make an account that you can use to follow people and try out the service.

What is Mastodon, exactly?

Mastodon refers to two things:

  1. Software that runs on a server that provides a microblogging service similar to (but distinct from) Twitter;
  2. A community of people who use those servers as a social network.

There is no single Mastodon, but rather any number of servers running Mastodon on different domains; as of this writing, nearly a thousand. For historical reasons (more on this later), Mastodon servers are usually referred to as “instances.” You can think of them as being a lot like email hosts.

Like with email hosts, users of a Mastodon server aren’t restricted to talking with users on their own instance. I have an old personal email address on, and a newer email address on a domain I own,; both can send and receive email to any email address anywhere on the internet. Similarly, Mastodon users can share posts and follow users on other Mastodon servers.

Also like email, your Mastodon client (your web browser or a mobile app — more on those later) isn’t talking directly to any server other than your own instance; the instances propagate posts and events among themselves, sending and receiving on behalf of their users. This concept is what is known as federation. When I make a post from my account on, that server sends it out to everyone else on that follows me; but it also sends it out to other instances that “follow” me because one of their users does.

Mastodon is open-source software; anyone can install it on their own server and run their own instance, whether that’s just for themselves and their friends, or for whoever wants to join. Similar to email, Mastodon accounts are named in a @user@host.tld format; for instance, I am; the initial @ distinguishes it from an email address. But users on the same instance can refer to each other by just their usernames, so you can mention me on by just using “@brunodias.”

The universe of servers talking to each other is collectively known as “the federation” or (again, for historical reasons) “the fediverse.” Despite what that terminology might imply, there is no centralized control of the federation; servers are totally independent from one another, and relationships between them are one-to-one, without some kind of central broker between them. It’s really more of a confederation., specifically, is the “flagship” instance operated by Eugen “Gargron” Rochko, who is also Mastodon’s primary (but not sole) developer.

Why would I want to be on it?

Depending on your social circle, you might already be getting dragged on Mastodon by your friends for not being on Mastodon.

More probably, you might be sick of Twitter’s constant tampering with the user interface, their shady use of algorithms to drive “engagement”, the sheer volume of advertising (I just scrolled down the last 20 minutes of activity on my Twitter feed, and Twitter inserted five ads into it), and the presence of fascists, harassers, and other asswipes on their service.

On a software level, Mastodon has:

  • Per-post privacy options;
  • 500 characters per post, instead of a measly 140;
  • A built-in content warning feature that lets users hide post contents behind a “see more” button;
  • A public timeline of posts, which can be useful to discover users worth following or just to see cool shit;
  • Decentralization.

That last point is important: different Mastodon servers can act as independent, autonomous communities with their own rules, which can be responsive to user needs. Problem users can be banned; if they’re on another instance, their posts can be blocked. If an entire instance of bad users crops up, other instances can “defederate” (block) it. Instances that are particularly concerned about user safety or creating a certain experience can work on a whitelist model, federating only with selected other instances.

And beyond the safety/privacy implications of decentralization, separate instances can also develop independent user cultures. You may find that the general vibe and habits in a particular instance suit you better; I like being in the big center-of-the-universe instance, but other users have a much better experience on smaller instances.

But Mastodon is more than the software;, and many other instances, were basically populated by some mix of furries, queer people, and radical leftists. Instances have since then popped up to serve various different communities and groups. In a lot of ways, Mastodon is less about mapping your existing social life onto an online space, and more about engaging with a third place.

How do I get on it?

Back when Mastodon was young, and had but 10,000 users, most people got on Mastodon by joining, and thus never had to think about “federation” or “instances.”

Currently, you can sometimes do exactly that: join through the form on the front page of that. But as demand outstrips the server capacity of a given instance, they might close for new registrations, as often is. Besides that, is huge, and so the public timelines on it are not as useful/relevant as they are on other instances. Other large, popular English-language instances include and

So, you can shop around instead for an instance. provides a fairly comprehensive list of instances, and it will also give you a random sampling of available instances to choose from, prioritizing ones that have open registrations and a good stability/security record.

Alternatively, I’ve curated a non-comprehensive list of instances with English-language users with specific/interesting rules and communities, see the end of this article.

Toots? Federated timeline? What am I looking at?

Most everything about Mastodon’s interface is more or less self-explanatory, but a few things seem to confuse or evade notice. So here are a few notes on that.

What the hell is a toot?

The noise a mastodon makes. On and as a default on other mastodon instances, posts are known as “toots” after a suggestion by an early Patreon supporter; hence the bright blue “TOOT” button. You might think reposting someone’s toot is known as “retooting,” but it’s actually called boosting.

The Public Timeline

Mastodon has a public timeline, so you can see a scrolling timeline of posts by all users. This is split into two timelines: the local timeline shows all public posts from users on your instance, while the federated timeline shows all public posts your instance knows about. For the most part, the latter means people on your instance, and people who have followers on your instance. It’s not a “global view” of the entire confederation (that doesn’t exist); it’s still curated by the users on your instance, though some instances use bots that automatically follow people to populate their federated timelines.

If you don’t want your posts to appear on the public timeline, you can set them as unlisted.

Post Privacy Options

The globe icon underneath the toot box lets you set privacy options for your post:

  • Public toots show up on the public timeline and are visible to everyone
  • Unlisted toots are visible to everyone, but only your followers will get them on their home feed; they won’t show up on the public timeline
  • Private toots will only be seen by your followers
  • Direct toots will only be seen by people mentioned in them

A few things to note about these: You can set your account to be private, which requires you to manually approve followers and is the main use case for private toots.

Also, please don’t take these as absolute or perfectly secure, especially if you’re on a bigger instance. Not all instances in the confederation run Mastodon (the software), and so not all of them respect privacy options in the same way; and the software is still evolving to live up to those options. You can think of those as affecting how discoverable posts are, not as a strict control on who has access to them. Please don’t treat Mastodon as a secure channel for private communication, in particular.

Content Warnings & NSFW Tagging

Mastodon lets you tag media you include in your posts as NSFW; this will hide it behind a click-through. Most instances ask that you do this if you’re going to be posting hardcore pornography or just something that might be disturbing to some, like that gif of the guy’s head exploding from Scanners.

The content warning feature, available by clicking on the little “CW” icon under the toot box, essentially splits your toot into two parts: an “above-the-fold” warning and the content of the toot, which users have to click through to see. Warning about potentially disturbing or personal content is the nominal reason for this, but in reality it has many uses. People use it to setup jokes, or to hide the body of very long toots. On, and therefore on many other instances, there’s a social expectation that users hide toots about current events, politics, or potentially anxiety-inducing topics behind a CW. Socially this is more or less equivalent to lowering your voice so that only interested people can hear what you’re saying.

Other Privacy & Safety Features

Mastodon doesn’t feature quote-retweeting nor keyword search.

Quote-retweeting has often been seen as a very straightforward way for assholes with a lot of twitter followers to sic their personal hate mob on someone; often, people with large followings do this unintentionally. Keyword search can facilitate someone going after anyone who tweets a certain word or phrase.

Does this mean it’s impossible for malicious users to use their followers as a harassment brigade, or that single-issue wonks can’t trawl through the public timeline and harass anyone talking about a certain topic? No, but the UI doesn’t go out of its way to facilitate it either.

If you include a hashtag in a toot, that will make it searchable by that hashtag; otherwise, there is no built-in search for toots. This doesn’t mean that hashtag-less toots are effectively buried; people can, and have, built third-party search engines that crawl Mastodon instances.

Mastodon is built with awareness of Twitter’s long and bloody history of harassment and consideration of these issues, but it’s not perfect and no piece of social software is ever going to be totally immune to malicious use. Being on an instance with available, friendly, and vigilant administrators helps a lot.

Wrinkles of Federation

Your instance doesn’t know about every single post and user in the entire confederation. This means that your federated timeline is in some way curated by the user base on your instance, and doesn’t represent the whole universe. But you can still follow users that your instance doesn’t know about by searching for their fully qualified user name (in the @user@hostname.tld format).

However, if you’re the first user on your instance to follow someone, you may not see their posts immediately as your instance works to fetch their posts and display them.

Overall, a thing to know about the confederation is that it’s a landscape, and you can’t see all points in it from the same vantage point, no matter where you stand. Power users who want to get a broader view might want to make more than one account on different instances.

Mobile Apps

Yes, there are mobile apps, Amaroq for iOS, and Tusky for Android.

Appendix A: Historical and Technical Notes for Nerds

Mastodon is a new implementation, written in 2016, of a standard called OStatus. OStatus itself is really a pile of other Web standards which, when brought together, enable the creation of federated social media services. GNUSocial is the older, traditional implementation of that standard, which harkens back to, a social networking service that launched in 2008 and has since migrated away from GNUSocial as its back end.

GNUSocial and Mastodon instances can talk to each other, and indeed, the confederation as it exists now is made up of both kinds of instances. GNUSocial however has a very different user culture, being populated mostly by developers and free software enthusiasts. And, of course, several GNUSocial instances are populated with members of various internet hate and fascist groups, either because those instances were set up explicitly for this purpose, or because said users set up shop and were never asked to leave. Several major Mastodon instances have now suspended those instances.

However, Mastodon currently dwarfs the old extant GNUSocial network; there are about 140 GNUSocial instances in operation, while Mastodon has nearly a thousand instances. As of this writing, in most ways that count, Mastodon is the confederation.

Appendix B: A Brief and Incomplete List of Notable Instances, in No Particular Order

  • is the original, “flagship” instance, and as such much of the culture on other English-language instances (such as their code of conduct) is descended from its culture.
  • is the original “whitelist model” instance; it federates only with a selected group of other instances chosen by hand.
  • and are other large, English-language instances that take after’s general vibe.
  • and are, in fact, the two largest Mastodon instances, having overtaken the flagship instance this week with a combined population of over 120,000 users; Mastodon has seen a significant influx of Japanese users. is run by Pixiv, a company that is essentially the Japanese equivalent of DeviantArt but which westerners mostly know as a sort of wellspring of anime porn. Due to the occasional posting of “loli” content on there (drawn images of sexualized minors, which are legal in Japan), pawoo has been blocked by a number of large instances in the West, for legal reasons if nothing else. By now some of that has been deescalated to silencing them from the public timeline and not caching their media uploads, a feature that wasn’t available last week when launched.
  • is a French/English-language instance populated by, well, witches. Registrations open daily at 8pm, French time, for a “witching hour.”
  • is like, but for hackers and cyberpunk.
  • forbids posts with that fifth Latin symbol, most common in normal Anglo writing.
  • eeeeee eeee eee eeeee e ee ee eeee.

I write for and about games relating to fantasy, science, the future, and politics. “The most dangerous person in interactive fiction.”