Facebook Email (or rather the messaging service) is Invite only and you can request the invite here.
The new Messages interface not only displays the Facebook messages you exchange with friends, but it also interweaves your chats, texts and emails (should you choose to create an @facebook.com address). It’s a central place to control all of your private communication, both on and off Facebook.
Now when you view a conversation, you’re actually looking at a complete history of all the communication you’ve shared with that person on Facebook — it’s like having an ongoing record of your friendship.
