I just learned about Facebook for Game Developers. I would like to know if I need to have the Facebook app installed in my phone to use Facebook for Game Developers (like I need to install Google play games services in android).
The game can use the Facebook APIs without facebook installed on the player's device.
I haven't used Google Play, but Facebook is unable to query the information of any friend that's not also using the game. Requesting the friend list will return an empty list, for instance, until another friend installs the game. There is an API for inviting friends to install the game, but I don't know how effective that is in the real world. (I've never accepted an invitation to play a game with a friend.) If you call the API that purports to send messages and requests to the user's friends (assuming the friends have not yet installed the game), the messages will show an invitation to install/play instead of the actual content.