In response to Bray’s toot, Evan Prodromou — one of the creators of ActivityPub, who is currently writing an O’Reilly book about the protocol — noted that this “is also the argument for using the ActivityPub API.” He described the API as “an open, extensible API that can handle any kind of activity type — not just short text.”
This gets to the nub of the issue. The fact that I can’t use my Mastodon identity to, for example, sign up to Pixelfed is not actually an ActivityPub issue — it’s because the two applications, Mastodon and Pixelfed, each require you to create an account on their respective products. What Prodromou is suggesting is that, technically, you can use the ActivityPub API for account access.
I can’t see how Apub’s C2S API can realistically be implemented. It’s fairly light on details and if I’m understanding it correctly the only standard way to get activity from the server is to pull from an actor’s inbox, which has to be an
OrderedCollection
of all the activity the actor has received (likes, notifications, posts, the lot). This shifts a lot of the work to clients which, apart from being being very classist, is very limiting for implementations.