Well, it is a problem with FB mail that once you mail a list of people, you can't add someone to the list unless you start a new thread. Normal people do want to do that.
They should just give users @facebook.com email addresses and put in a webmail UI.
Look, in VERSION 2 we added NEW features to our messaging platform that you all have known and loved for YEARS! In fact! since the freaking invention of email!
Also, given that the creator of gmail has joined FB, wouldn't he have said "uh, guys - I think we forgot something. guys??"