Facebook Launches One-Way Mailbox API.
techcrunch.com
techcrunch.com
My uneducated guess would be that ads on gmail don't perform all that well, but if facebook could figure out how to monetize messaging that would be huge.
It's one-way because Facebook has to be worried about spam since anyone can send a message to anyone else. Twitter doesn't have this issue because you have to opt-in before you ever receive a message from someone.