I unfortunately don’t know how the numbers break down. However, I see a lot of people that use Facebook for groups. As a result, I wonder how much of their engagement is driven from people posting inside of various groups that might be private.
It might lead to the situation that OP describes where most of the “forum” conversation has died down but it’s thriving in various segments.
Single data point here. I haven’t actually used FB regularly for 5+ years. When I was single, there were a few dating apps that required it so I kept it around for that reason. Currently the only two features that are even remotely tempting on are groups and marketplace. The latter seems to have taken over the role of “classified section” for the internet from Craigslist so I feel like I’ll have no choice except to use it on occasion. But yeah, groups seem increasingly important to FB for locking people in to their site.
It might lead to the situation that OP describes where most of the “forum” conversation has died down but it’s thriving in various segments.