What you are referring to as ActivityPub is actually Mastodon/ActivityPub or as I've recently started calling it Mastodon + ActivityPub. ActivityPub is not a functional social media protocol unto itself, but rather another component of a fully functioning Mastodon network made useful by the Mastodon backend, Mastodon frontend and vital extensions comprising a full social media protocol as defined by Mastodon. Many social media users run a similar implementation of the Mastodon protocol, without realizing it. Through a peculiar turn of events, the version of Mastodon protocol which is widely used today is often called "ActivityPub", and many of its users are not aware that it is basically the Mastodon protocol, developed by Mastodon. There really is an ActivityPub, and these people are using, but it is just a small part of the network they use made by cwebber and her friends.

ActivityPub is the core: the base protocol in the system that established the base for our extensions. ActivityPub along with Activity Streams 2.0 is an essential part of the Mastodon network, but almost useless by itself; it can only function in the context of dysfunctional social media. ActivityPub is normally used in combination with the Mastodon extensions: the whole network is basically ActivityPub fixed by Mastodon extensions. All the so-called ActivityPub implementations are really Mastodon/ActivityPub.