The problem with this? (and all other attempts to change "messaging")
Is that it requires users to do something different than they expect or do currently. The goal of having a "unified inbox" won't work unless you natively integrate with all the places where somebody currently sends messages. This requires the user to change their behavior for what? To have a "unified inbox?" I don't think that's compelling enough for user to change their messaging behavior.
For many people that won't be a problem - I think of those who already use Facebook inbox regulary, many of them already more often than e-mail. I've found out myself doing it more often recently. Why? It's easier and more convenient - when I write a message to John Doe I know exactly to who I am writing, with e-mail you have to remember which e-mail was whose, or which of the e-mails of Joe Doe that you have is that correct, latest one; and the same with phone numbers and SMSes. Yes, in a big scale of things, moving your communication with others to Facebook isn't the best idea ever, but in a day to day hurry you often forget about the great, big things and just stick to what's seems easier and faster now. That's why I think this has all the chances to actually be successful, for better or worse.
Yes, it'll be easier. But I, for one, won't use Facebook Messages for SMS. Why would I text Facebook to text my buddy for me? When I can just select him from my address book?
All of a sudden the idea of unified communications and messaging is out the window.
Is that it requires users to do something different than they expect or do currently. The goal of having a "unified inbox" won't work unless you natively integrate with all the places where somebody currently sends messages. This requires the user to change their behavior for what? To have a "unified inbox?" I don't think that's compelling enough for user to change their messaging behavior.
Google has the same issue with Google Voice.