Facebook is completely rewriting their messaging product and is preparing to launch Titan, a fully featured webmail product in its place.
FB has 400 million users.
Titan will offer full POP/IMAP: you can can access the account other than through Facebook. Your email account name will be firstname.lastname@example.org.
Quite simply, if Titan can sort emails, I'm switching. GMail's primitive interface is a hassle.