why reinvent the wheel badly?

They should just give users @facebook.com email addresses and put in a webmail UI.

That makes it too easy to communicate with non-facebooked rest of the world.

Actually, that would be great for facebook. Sending millions of emails per day to the outside world with a link 'XXX is already on facebook, why dont you join today' at the bottom would be priceless.

Well, yes and no. A major advantage of FB is that, once they suppressed the crazy app invites, it's spam-free. The only people who can send me messages are people I've already "whitelisted" by adding them/accepting their add. Once they start allowing inbound email, it all goes a bit hatstand.

Why? You'd just have 2 boxes - "Email from your FB friends", "Email from other people".

That's exactly what they have done with the new version of messages.

But isnt that just plain retarded?

Look, in VERSION 2 we added NEW features to our messaging platform that you all have known and loved for YEARS! In fact! since the freaking invention of email!

Also, given that the creator of gmail has joined FB, wouldn't he have said "uh, guys - I think we forgot something. guys??"

It would have to be a heavily optimised webmail UI. Current webmail implementations are too heavy on legacy features that normal people don't use (BCC/CC), have issues with spam ("why do good emails end up in my spam bin where as this viagra ad does not?") and light on social link and media sharing features that facebook users would expect.