Note: Only available in the US at the moment.
With Google+ coming out to compete with Facebook, it came out with a cool feature called "Huddles" which was a messaging platform solely for talking to people on Google+.
Facebook has now come out with its very own version of "Huddles" called Facebook messenger which is available on Android & iOS. The app ties in with your Facebook account and inbox. So say your search for a friend, when the results come up, some of them may have a phone icon by their name meaning when you send them a message it will be sent as an SMS to their phone. This will only work for those who have entered their mobile numbers into their Facebook profile.
Facebook messenger is a separate app just like the Google+ huddles and is completely free.