I'd just like to interject for a moment. What you're referring to as Mastodon, is in fact, Fediverse/Mastodon, or as I've recently taken to calling it, Fediverse plus Mastodon. Mastodon is not the Fediverse itself, but rather another free component of a fully functioning Fediverse system made useful by the ActivityPub specification, its various implementations and vital system components comprising a full Fediverse as defined by ActivityPub.
Many computer users run a modified version of the Mastodon system every day, without realizing it. Through a peculiar turn of events, the Fediverse which is widely used today is often called "Mastodon", and many of its users are not aware that it is basically the Fediverse system, developed by different people.
There really is a Mastodon, and these people are using it, but it is just a part of the system they use. Mastodon is an implementation: the program in the system that allocates all the machine's resources away from other programs that you'd try to run. Mastodon is an essential part of an Fediverse, but useless by itself; it can only function in the context of a federated network. Mastodon is normally used in combination with other ActivityPub software: the whole system is basically Fediverse with Mastodon added, or Fediverse/Mastodon. All the so-called "Mastodon" servers are really instances of Fediverse/Mastodon.