I don't have numbers unfortunately, but with Facebook what I think also came was "everyone else". People on forums and chatrooms were still in a niche group of individuals who cared enough about a niche topic AND cared enough to have a PC and an internet connection. When smartphones came along, we were getting everyone online, and the network effect of a platform like Facebook meant that even people who would have preferred forums had to go sign up to Facebook to stay in contact with social groups they were part of. Forums couldn't compete with the number of new groups and communities being formed on Facebook, and the pull of that network effect.

For some reason a lot of specialist car specific forums have managed to stick around, I think because they have long functioned as knowledge bases, and Facebook does not function well for that use case.