Nostr is another protocol that is growing rapidly especially since Jack Dorsey discovered it a few days ago and is pushing it hard on twitter. He even made a 15 btc donation to its development fund.
Nostr is a more sane version of activitypub where your identity is just a public key and not dependant on any server, no signup required. Due to this it also natively support BTC payments and micropayments through the lightning network