Facebook now allows users to send e-mails through its messaging system. So if I want to share a link or send a party notification via Facebook, I can just tack on my non-Facebook-using friends' e-mails to the end of the To: line and they're in the loop, too.
A few thoughts:
A) It's a crafty (and sneaky) way for Fbook to pull more users. If you've held out this long and you're getting e-mail messages from it, you'll probably just end up joining the damn thing. This tears down the barrier to entry for those people who go, "Oh, I don't know how to get me a Facebook thingy."
I tried this out with another e-mail address I have. I sent myself an e-mail from Facebook then opened the received message in the aforementioned e-mail account. I was directed to a Fbook page with the ability to see the entire conversation and respond as much as possible - all without logging in to Fbook.
B) If used, this will keep Facebook at the focal point of users' browsers - surely what it (and advertisers) want.
I'm just waiting on the instant message client to be announced...