For questions specifically about the Facebook Messenger standalone app.
It was originally released in August, 2011. In December, 2012, the app was updated to allow use with only a name and a phone number (to act as a replacement for SMS) in a few countries. It was later updated to allow the app to also act as a SMS client replacement. In April, 2013, coincident with the launch of facebook-home, the "chat heads" feature was added, which keeps a round icon overlay for a chat conversation active even when using other apps.