The nice part of the new Facebook messages product is that it isn't a standalone or competing messaging product, unlike many other products you are alluding too. It unifies SMS, email, and IM, and it interoperates with all of them. So if your friend emails you, you can reply by SMS or by IM, and they will receive it back in their email inbox. Likewise, if your friend is online when you email them, it will pop up like an IM, so they can reply in real-time.

I think this type of product has been a growing need for people for a while. Literally every single one of my younger cousins only uses SMS and IM to communicate and every single one of my older relatives uses only email, so you end up having to choose a communication protocol based on recipient rather than just choosing a person and composing a message.

It still has a number of quirks, but I personally think this is a really nice product. (I am admittedly biased, but I am being honest.)