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, ActivityPub plus Mastodon. Mastodon is not a social network unto itself, but rather another free component of a fully functioning ActivityPub system made useful by the ActivityPub namespaces, vocabulary and non-normative sections comprising a full social network as defined by W3C Recommendation 23 January 2018.

Many computer users use a modified version of the ActivityPub social network every day, without realizing it. Through a peculiar turn of events, the version of ActivityPub which is widely used today is often called "Mastodon", and many of its users are not aware that it is basically the ActivityPub social network, developed by the W3C.

There really is a Mastodon, and these people are using it, but it is just a part of the social network they use. Mastodon is the frontend: the program in the system that lets you interacts with the posts of all the people you follow. The frontend is an essential part of a social network, but useless by itself; it can only function in the context of a complete social network. Mastodon is normally used in combination with the ActivityPub social network: the whole system is basically ActivityPub with Mastodon added, or ActivityPub/Mastodon. All the so-called "Mastodon" distributions are really distributions of ActivityPub/Mastodon.

@mewmew "Mastodon is the frontend: the program in the system that lets you interacts with the posts of all the people you follow. The frontend is an essential part of a social network, but useless by itself; it can only function in the context of a complete social network."

I don't believe that's right. As I understand it, "Mastodon" does not refer solely to a frontend but to a codebase with multiple parts including both a frontend and a backend. And even that distinction is sketchy.