@Stark There is, for example, #Zot. Zot was created along with a #Friendica "fork" (actually almost complete rewrite) named #RedMatrix which, upon its 1.x release, become #Hubzilla.

I don't have the exact technical specs, but it should work similarly to #ActivityPub. It's much much more powerful, though.

For starters, Zot was created for something that goes even beyond federation, namely #NomadicIdentity. Not only does it facilitate the move an entire channel from one instance to another, it lets you have your channel on multiple instances at once and automatically keeps all copies in sync. If the hub with your main channel on it goes down, doesn't matter, you have an identical clone or several.

Besides, Zot was not only designed for messaging. After all, Zot can keep channel on that monster named Hubzilla in sync. With everything on them. Not only posts, entire threads and contacts, but articles, wikis, notes, the content of your WebDAV-equipped cloud file storage, your public event calendar, your private CalDAV calendars, your private CardDAV address book etc.

This gets really interesting if you're on Hubzilla, and you have another Hubzilla channel amongst your contacts. That channel may have a number of nomadic clones, but you only see one of them as the main one. The channel owner can seamlessly change which one is the main, and all you notice is that the hub URL has changed.

The current and last version of Zot is Zot6, originally developed for and with the Hubzilla successors #Osada and #Zap, then backported to Hubzilla.

#Streams, basically a code repository which evolved from Zap through another two steps, already uses #Nomad, a kind of successor to Zot which nonetheless can communicate with Zot. Streams shows what Nomad is capable of: Instead of a "fully-featured" server platform, Streams is meant to be a code base for advanced Fediverse projects, i.e. you can strap onto it whatever you want to develop. Whatever it may be, Nomad can keep it in sync between multiple instances.