There are a lot of alternatives popping up: BlueSky, Mastodon, Farcaster, nostr. There are also some older protocols that people are still building on like secure scuttlebutt.
I've written about some of the differences of these protocols here:
That was a good read for me - someone who has heard of Mastondon and ActivityPub but hasn't really grokked the tech and definitely didn't know other alternatives.