I understand where you're coming from. However consider this. I am a member of car, body building and few other forums out there. Every single one is down 70-80% and yet their FB groups are up and growing like exponentially. I am also a member of a millionaires club. They talk about all current business trends. Every single one has said the same thing.I don't think Facebook is the issue. Almost every top poster and other members have left here pissed off for some reason or another.
Lastly, FB targeted "forums" to gain market share. They made it user friendly, super fast and a one stop shop for personal and business. To say they have been successful is a major understatement. With think group tanks they made it easy for anyone to start a "forum" per se and take members away from forums. It was genius, or disingenuous depending on your viewpoint. They have made billions while building a monstrous platform, dismantled most forums in the process and now have control over most thought. No wonder they are one richest platforms on the planet.