It should probably have gone:

Centralized

Peer to Peer

Federated

ActivityPub is just one federated protocol. There are much older ones, like email. Some newer ones would be matrix (dm, voice and video, basically does what whatsapp/imessage/telegram/discord does, but federated) and diaspora (similar to ActivityPub in function, but made much earlier).

And there is always the first. Usenet, which was, and still is, kinda like BBS and forums, but federated!