So... partaking in the 'fediverse' - whatever that stands for - requires signing up. Now I'm wondering two things: 1) How is that different than FB or Twixxer? And 2) Why not use RSS, OPML and the open web that's already like... open?
ActivityPub has an outbox and an inbox. The outbox looks a lot like RSS, and allows for publishing things. The inbox allows you to receive interaction. Likes, replies, boosts, etc...
RSS works without authentication (though you can require it) because your server doesn't have to know who is following you or how to notify them. ActivityPub is an attempt to add pub/sub to RSS, they actually started with many of the same data models from RSS and its successors if I'm not mistaken.
Combine that with ActivityPub being federated rather than peer-to-peer and you end up with a lot of legal gray area to go along with the added complexity and loss of anonymity.