Facebook (and Twitter to some extent) solve one of the biggest problems of email which is the concept of verified sender.

I add it to the paradigm shifts as it resolves (in its own [large] namespace) a longstanding problem with email.

I add it to the 'transient' list as its solution is purely driven by network effects which leaves it vulnerable to the next player sideways market dissolution.

It's a bit of a stretch to say they solved it. I'm not on FB and so FB to me don't matter. On the other hand, I'm part of an Active Directory of my company, so AD solves this problem for me at work. But none of them guarantee that the email came from the particular person and not from a dog.

No, Facebook Messages and Facebook Chat are two different things.

http://www.facebook.com/about/messages/