I’ve begun to wonder about this question. There’s nothing implied in the existence of the fediverse that it will include every site, and there’s nothing implied in the existence of every non-fediverse site which would stop them from forming their own fediverse. One could technically say email providers have already achieved this, though this only really applies to email providers which doesn’t make it as useful as the fediverse we know and love. Are there actually other fediverses out there?
Fediverse to me means a protocol, the technical term would be ‘ActivityPub’ protocol, which all these servers use to share data between themselves… In that sense yes… there are countless fediverses/protocols… one that quickly jump to mind is the Blockchain based protocols, another is Bittorrent protocol, however non of them comes close to what ActivityPub has achieved, I would say the only one that did come close, and actually surpassed activitypub in some aspects is the Nostrverse which is really technical in comparison, it has it’s drawbacks… both of them does…
another is the hypertext transfer protocol… (?!)