There is the ability to add private notes. For example, when I selected the above text, Medium created an edit box with a “private note” option at the bottom.
Medium makes it clear that the notes are not completely private, since apparently the Medium admins can see them.
The private notes are somewhat awkward. It is easy to accidently remove them. The first note is limited to 400 characters and subsequent notes are limited to 200 characters (shades of Twitter).
I have been considering a messaging service for nderground. A full fledged private messaging service is a fairly big deal. Medium presumably has the resources to do this, but there is a lot of design and implementation work that would go into this.
Medium made some design decisions that I would not have made. They made a big deal about implementing their own browser based editor.
For nderground I made a different choice and licensed the Redactor editor, which supports a much richer set of typesetting features.
One of the reasons that software implementation schedules are so difficult to get right is that there are features that can be explained in under five minutes that can take months to build. Even for software engineers there’s a temptation to think “How hard could it be?”