I'd like to interject for a moment.

What you're referring to as #Mastodon is in fact the #Fediverse.
Mastodon is not a network unto itself, but rather another open component of a fully functioning network made useful by #ActivityPub.

Every Mastodon user is in fact a user of the fediverse without realizing it.
There really is a Mastodon, and these people are using it, but it is just a part of the greater whole. Mastodon is one example of a service using the AcivityPub spec, and is an essential part of the Fediverse, but of limited utility by itself; it is best understood and functions at its greatest when used in combination with other Fediverse services.