Technically speaking, the #Fediverse includes all #federated instances of all projects that use certain protocols. But it's hard to say where the borders are.

It's true that the Fediverse is more than just #Mastodon. Many Mastodon users still don't know that because they haven't heard of any other Fediverse project yes, some even reject it and want the Fediverse to be Mastodon and Mastodon only.

So for starters, the Fediverse contains everything that uses #ActivityPub, that has federation on, and that isn't widely defederated/Fediblocked.

This also includes services which don't have ActivityPub as their core protocols, but which understand them: #Friendica (core protocol: DFRN), #Hubzilla (core protocol: Zot6), #Streams (core protocol: Nomad). Whereas Friendica and Streams have ActivityPub always on, it's optional on Hubzilla, both hub-wise and channel-wise. Most hubs have it installed, though, and most channels have it on.

By the way, if you say that Friendica is not in the Fediverse, then this entire thread doesn't happen in the Fediverse. @Fediverse News runs on Friendica.

Now it gets complicated: Is #Diaspora (the "Facebook killer" spectacularly crowd-funded in summer 2010) part of the Fediverse, or isn't it?

Diaspora* is decentralised, distributed and federated all within itself. Still, it was designed as a walled garden. Diaspora* was never meant to connect to anything else. It's like XMPP without gateways or Matrix without bridges.

Then came Friendica which, at that time, had the goal to federate with just about everything that moved. Diaspora*, still an alpha release, didn't have an API. So the Friendica devs reverse-engineered Diaspora*'s protocol, developed a connector and federated Friendica with Diaspora*.

To this day, not much more than Friendica and Hubzilla can federate with Diaspora*. But you may still find a post from a Friendica user in your Mastodon timeline with a comment from a Diaspora* user. Diaspora* doesn't federate with Mastodon, but that comment went from Diaspora* via Friendica to the ActivityPub-speaking parts of the Fediverse.

So on the one hand, Diaspora* is connected to projects that speak ActivityPub. On the other hand, this has never been Diaspora*'s own development or achievement, and it doesn't directly federate with Mastodon or any other Fediverse project, save for those select few.

Besides, if Diaspora* counted as part of the Fediverse, then so should e-mail, WordPress and everything that uses RSS. Friendica and Hubzilla federate with these as well. Those of you who follow me should find in their Mastodon timelines two automatically reposted blog posts per week from a blog which I've subscribed to via RSS. Hubzilla can do that.

Speaking of Hubzilla, entire hubs without ActivityPub are another edge-case. They don't connect to any projects that only know ActivityPub. However, they, too, can indirectly communicate with ActivityPub instances through either Friendica (by means of the Diaspora* protocol unless that's turned off, too) or Streams-based instances (Streams' Nomad is basically a newer version of Hubzilla's Zot, so both speak a common language).

Nonetheless, I dare say there's no denying that Hubzilla itself is part of the Fediverse.