It's the underlying protocol for Mastodon, PeerTube, and a few other things. Right now it's the darling of the distributed social networking crowd (because there's a lot of well-deserved reactionary attitudes towards centralized social networks) but the reasons it's caught on instead of a resurgence of Atom or whatever feel a bit... contrived.
The main things I hear from ActivityPub folks is "it's a W3C standard!" (yeah, and? Atom is an IETF standard and its components are also part of W3C recommendations?) and "everyone's using it! RSS died when Google Reader did!"
Also I feel like the fact Atom is XML with a fairly strict schema while ActivityPub uses JSON as its primary serialization spec might have a lot to do with it. Even though the actual nuts and bolts of serialization don't really matter, and there's a lot to be said for self-describing data structures and so on (and not to mention that Atom also functions perfectly fine in a static publishing context and tolerates failure better and don't need a lot of resources to scale based on subscriber or subscription count and so on). But whatever.
Obviously I could rant about this quite a lot, and in fact I have on my blog way too much, especially lately. :)
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