E-mail would be part of how different social networking sites interoperate.
E-mail is a patchwork of proven standards.
E-mail might vanish into the plumbing of the internet --- as ip addresses have and urls might --- but it will still be there.
If my company got rid of e-mail altogether and used Facebook instead, we'd lose customers.
Catchy titles grab attention, but they obscure meaning. I still don't know what you are predicting.
i use email now only because it's a legacy standard though this is definitely decreasing in favor of social apps. this is analogous to television and internet.
my prediction is that a social app will take over email as the primary internet communication method and you will start to see this manifest itself very soon, even inside of corporations.
i think social app communication features are much more usable than email, there's also a linkage between users in social apps that can be leveraged to provide even higher levels of usability: CRM integration with contact management, calendar, customer support, etc.
imagine that your company got rid of email altogether and used something like a facebook app for all communications.
i view facebook and social apps as a communication tool with multiple dimensions as compared to email, a social app being a superset of email. so why carry forward email, other than that it's a legacy standard being used by a lot of users at the moment.
imagine business cards of the future:
Name
Social App ID
there would obviously be some time before this would effectively take hold of the greater amounts of users, but i think that time is soon.
if facebook added an administrative interface and made it multi-tenented, the pitch could be to throw out your exchange server over time. i'm not sure what the worth to their strategy would be in the future to take on enterprise users, but i imagine it's quite valuable.