You can call it "the federated social web" or whatever you want. ActivityPub is the name of a protocol. Regular people talk about e-mail, not about "POP3", etc.
By the way, I don't recall Diaspora or Friendica people commiting to implement this, but the thought of Twitter and Facebook becoming part of it, is quite amusing xD”
You certainly don't seem to understand what I want to say, and most probably I don't understand important facts about "ActivityPub".
The whole point is you CAN'T call it anything you want. Email is the best example. You call it "email", not "digital letters" or "electric post" or whatever.
I seem to be ignorant about what to expect from ActivityPub though: I was hoping to one day ask people if they are on "ActivityPub" to follow them - or not. The way you describe it is like it being some kind of matrix, where it could be possible to have gnusocial and diaspora using activitypub but still being incompatible. If they would be compatible - what would be the point of callig them either way instead of "ActivityPub"?