There’s also Matrix, which I think is garnering more users. Matrix is still in only a beta grade level of maturity, however. I would suggest it’s best left to geeks at the present time. For those who might want to run their own server, it will need to be a beefy machine, as Matrix is currently written in Python, and it isn’t very efficient code at present.
Also worthy of mention is Mastadon. It’s an Open Source clone of Twitter. A Mastadon server can be federated (available to the entire world, to receive posts), or private, at the discretion of the server admin.
Be warned that if you run your own federated Mastadon server, you will need moderators who actively block the garbage posts out of the server (like porn, spam, etc). Suttacentral Discourse already having moderators of its own is a great resource, I tell you.
I think an important takeaway here is that as soon as you have some sort of self-hosted server, which is public to the world (AKA “federated”), you had better have a team of moderators to go with that! My Mattermost server, by contrast, is private, so the moderation effort is much less: each new user needed access to an “invite link”, which I allow and deny with discretion up front before new user account creation.
An analogy here is that you’ll either need “bouncers” inside the nightclub (in the case of federation), or bouncers outside, by-and-large guarding initial entrance to a private nightclub (in the case of a private server). “Bouncer” here is analogous to moderation effort by humans, and “nightclub” is analogous to a server with some sort of forum/communication software on it.