Seems like Facebook is serious about its Messenger app. Just today, they have notified users from a few European countries that soon they wont be able to message their friends on the Facebook app anymore. The chat icon will still be in there but when tapped, it will force users to the Messenger app.
Actually, I have already been experiencing this on my device. Messages opened in the Facebook app launches the Messenger chat heads.
Note that alhough this change will be rolling out globally, lower-powered Android devices will still be able to use the in-app messaging system due to hardware restrictions.
Messenger app by Facebook is currently available in the App Store, Windows Phone Store and Play Store for free.