I'd just like to interject for a moment. What you're referring to as Mastodon,
is in fact, ActivityPub/Mastodon, or as I've recently taken to calling it, the Fediverse. Mastodon is not a social network unto itself, but rather another free component of a fully functioning federated system made useful by the ActivityPub protocol, JSON-LD and Activity Streams.

Many computer users run an implementation of the ActivityPub protocol every day, without realizing it. Through a peculiar turn of events, the implementation of ActivityPub which is widely used today is often called "Mastodon", and many of its users are not aware that it is basically using the ActivityPub protocol, standardised by the W3C.

There really is a Mastodon, and these people are using it, but it is just a
part of the network they use. Mastodon is one implementation: the program that relays posts and other activities between other users on the network.
Mastodon is an essential part of the fediverse, but less useful by itself; it functions best in the context of a complete federated network. Mastodon is
normally used in combination with the rest of the federated network: the whole system is basically ActivityPub with a few extensions added, or ActivityPub/Mastodon. All the so-called "Mastodon" users are really users of ActivityPub.