We've already seen apps like this before. Kik Messenger is one of them, if not the most used one, specially for iPhone users. The adoption however within mobile users was low. Facebook didn't even tried to do that but failed because its Facebook Groups is not as good as Google+ Circles.
And why will Huddle succeed where Facebook and Kik Messenger failed before? First because Google is Google, and everybody has a Google account. To start a group chat on Kik, all of your friends must sign up first, hence the problem with adoption. Something Google don't have to worry about. Second, because Facebook wants - needs - to have all information its users publish be public available.
The Huddle app, although very similar to Kik Messenger, has greater potential.