Soo... basically whitelisting functionality. Something that we've had for a long time. Only this is automatic since people are (hopefully) more diligent about their Facebook friends list than their Gmail/Hotmail/Outlook address book.
That's my SWAG, although you can go a little further than your friends list given a social graph. You can score an email based on all sorts of social data. Are you friends? Friends of a common friend? Do you have "likes" in common? Do you belong to common groups? Have you commented on the same thing? And so on.
Any large email player can do some of the same things with a history of emails, but FB could (I am only guessing) do interesting things.
Whitelisting is one approach to the spam problem. Inbox prioritization is an interesting problem as well. Once we have emails in your message box, you could log into FB and find 100 or more emails even after filtering for spam. So... Which emails should appear on your home page and which should wait quietly in your inbox for you to take action?
