I think we need multiple protocols. One for ordinary messages (ok, we have email for that), one for statuses, one for tagging people, one for publishing events, one for indicating which events you are going to, etc.
No, you just need to handle few additional smtp header fields ;-)
The thing about multiplicity is that most people aren't glued to multiple applications. The ideal for someone not putting a lot of effort to computers themselves is to just have a single window where everything of interest comes through. A single application that melds together multiple protocols would nice. But then you're talking something that seems like Facebook to the end-user but has the delivery mode changed.
The important issue is that we might want different identities for every protocol. Right now, lots of people prefer to have different identities on youtube than on gmail.
Instead of segregating identities on services I think a better model would be just a good support for multiple identities (which then can span between services).
But you don't want one service being able to link all those identities together.