There's always a trade off for just how agnostic you can be, so there's no way to support every possibility. But if you look at the different federated social web projects, there are clear similarities between them (ie, identity is in a username@domain format), so you can easily build around those similarities.

For instance, building around events ("posting a status update", "sending a friend request", "verifying an identity") and abstracting out those events into hooks is how Appleseed does it.

I have no misconceptions that this will hold up to every possible social protocol, past, present and future, but thinking about the protocol as an abstraction, and building the platform out from there, has already shown itself to be very useful.