Facebook has become this scary entity that has really swallowed up communities. I see new groups pop up on Facebook that eclipse forum communities in both membership and activity.
BUT... most activity on Facebook is a flash-in-the-pan. Discussions I see are quick thoughts or image posts with little content that get some quick likes then fade. Any meaningful communication can get buried quickly. With forum communities old convos can rise back to the top and active threads can keep going.
All these people putting time into building communities on Facebook are doing it for someone else! You don't own your data, can't predict changes in the system, or even build what you want. It is something that has grown out of convenience.