Can't you just send someone an email to their facebookid@facebook.com since most people have a facebook email address and don't know about it? And then just use a fake from address of whatever@facebook.com?
This sends the email to the login email address, not to the @facebook.com address, you could fake the email headers but that'd get caught by most webmail filters as a spoof.
I think theoretically the @facebook.com email address is now supposed to redirect to primary email too, even though that doesn't really work for me (bounces back)
