This feels so incredibly abstract (again). There’s a real-world problem I’m trying to figure out: how to know ahead of time what kinds of activities a given (S2S) inbox is capable of processing. That is, whether the software running on a remote server has the concept of a friend request, or whether it supports walls, or groups, or photo albums, and so on. This is required to make a UX that is convenient and frustration-free for everyone. There needs to be a robust capability negotiation mechanism. There also needs to be a way to specify privacy settings for active actions, so that e.g. only the users that can actually post on a wall see the form to do so.
Theoretical abstract concepts are nice, but I wish we talked more about the actual bits on the wire because that’s what everyone ultimately ends up dealing with.