IMAP are POP3 optional. /var/spool/mail/$USER or ~/mobx were enough some time ago, although right now not everyone is running their own mail (smtp) server.
The good news is that lots of people are going to be able to send and receive email messages using email adresses.
Both these messages and adresses are rfc2821 compliant.
By Metcalfe's law, this can only increase the value of the email network.
Remember, first AOL was an island, then AOL offered an email gateway, and soon enough AOL became irrelevant.
I don't know if the same is going to happen with Facebook, but if it happens, I won't be surprised, for Facebook is like a second wave AOL, a much bigger wave than the first one, because this time it's like "everybody" participates, not just the early adopters.
To me, an email is basically an email address.