About Matrix Matrix is an open protocol for decentralised, secure communications.
Matrix Manifesto We believe:
People should have full control over their own communication. People should not be locked into centralised communication silos, but instead be free to pick who they choose to host their communication without limiting who they can reach. The ability to converse securely and privately is a basic human right. Communication should be available to everyone as a free and open, unencumbered, standard and global network.
While it’s true people don’t say “I’ve joined ActivityPub”, isn’t that synonymous with “I’ve joined the Fediverse”? Besides, the organization behind it does market it that way — they themselves refer to it as “joining Matrix, using one of these clients” (Element, Fluffychat, etc). Like, that’s what their website is called, and so is the Matrix server they host.
Their centralization is, I think, a little more advanced than Mastodon’s. The organization that maintains the protocol regularly adds features to it, and then of course immediately updates their own client and server implementations to have those same, recently added features, meaning the other client and server implementations are always behind on at least a few features. It’s becoming reminiscent of how the web browser spec is so bloated, and gets new stuff added to it with such regularity, that new browsers are basically impractical.