While Facebook already has some basic email functionality, Arrington suggests that Facebook could soon introduce a complete web email service (like Gmail?):
First, our understanding is that there will be full POP/IMAP support, meaning users can access the account other than through Facebook itself. Your email account name will be your vanity url – vanityurl@facebook.com.
Email is all about identity. And Facebook is ahead of everyone else in the identity game via Facebook Connect.. Tacking a real webmail product on top of those vanity URLs and Facebook connect is something even Google may shudder at. link.