I'm almost convinced at this point if somebody simply built a facebook like interface, and used email as the messaging protocol, but didn't display it like an email, more like a FB conversation (as opposed to a threaded or gmail-like conversation view), that it would be used. There's not really much that's offered by alternate messaging solutions that email can't do if it existing in the right sort of framework:

1) Person-to-person? Check

2) Person-to-group? Check

3) Group-to-group? Check

4) Group-to-person? Check

5) Multimedia? Check

6) Fancy formatting? Check

7) Attached files? Check

8) Near real-time? Check (most of the time)

9) Deliver Fault tolerant? Check

10) Federated/distributed? Check

Why everybody seems to want to reinvent email is beyond me. About the only thing that FB or G+ offer is the storage of a server-side contact list so I don't have to remember their email address and the use of a white-list for sender/receivers so I don't get spam. Offer the same with email and a decent web interface and you've basically recreated any "modern" social network, except it'll be built on a time-tested, robust protocol that's already ubiquitous across the internet and already has billions of users.

edit I guess what I'm saying is that it's the application layer (server and client side) in the email equation that needs most of the rework, not the protocol layers.