Facebook announced its new Titan Messaging System (aka Fmail) on 11/17.
In an official blog post, Facebook software engineer Joel Seligstein explained that the service is different from email as we know it:
"There are no subject lines, no cc, no bcc, and you can send a message by hitting the Enter key. We modelled it more closely to chat and reduced the number of things you need to do to send a message. We wanted to make this more like a conversation."
In the video below, CNET editor Rafe Needleman (of Red Herring-fame) tried it out and talks about the features.