In 2011, Facebook launched its Messenger application which moved call and messaging functions away from the main Facebook application into a separate one.
Now Facebook appears to be testing moving calling and messaging back to the main Facebook application, which assumably would replace the Messenger application. Facebook is testing this with several countries including the US. It is not known if it will become a permanent change or if it will be rolled out to more users or not, but they are at least testing the change.
It is unclear why Facebook moved calling and messages away from their main application originally. But we can assume the decision to test bringing these features back under one application is in the hope of boosting user interaction with Facebook services, with everything all in one place this it would likely do so.