These things are true — to a point. If you have a voice, you can use these platforms. But I’ve found that you need a center of gravity. A blog / full website is often a good place for that, because it offers something none of the others do: longer-form content. This makes it an excellent anchor for the promo on other sites.
As an additional and very important bonus, your blog can be fireproofed from anyone else’s whims re: continued presence & operation. If you run and host it, you get the benefits of ownership as well as the chore. Which means the center of your marketing is truly secure over the long term. This is not so at Twitter, for instance, where your entire presence can be suspended due to griefers or trolls.
Your anchor doesn’t have to be a blog or web site, but your online voice should definitely have an anchor. Choose it wisely.