They're not offering email; they're trying to replace email with their own platform.
Email is an open distributed platform where everyone can set up their own mail servers and have control of their own mail. There are tens or hundreds of thousands of email systems out there. To replace that with a system whereby one company has complete control of your messages? Bad.
This seems to me to be a long overdue and obvious overhaul of their crufty messaging system based on the realization that many people actually do not use e-mail anymore and rely instead on Facebook’s woefully inadequate messaging tools.
To me, an email is basically an email address.
IMO "email" is a combination of the various transport technologies and the message format.
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.