Since update 6.9.6 Facebook app must be installed on Android devices to be able to log into Facebook in the game. It is caused by changes on Facebook - they stop supporting login via web form - pop-up window that appears after tapping on the login button. 

Open Facebook app and log into Facebook in it and only after that open the game, then open Settings and tap Facebook not connected button to log in.

If the Facebook app is not installed on your device yet, then it can be found in Google Play store. If you experience any issues installing it, please refer to Facebook support pages.