Facebook provide a medium that appears to be useful. Useful to Facebook.
We have a local Facebook page that the owner trumpets as being vital giving information about the town. But it is membership only group, so no one else gets to see it, and he arbitrarily bans people (like, for example, if they run a business that competes with his). Then on the group things like threading and duplication are very badly handled, and Facebooks algorithm gives you whatever posts it thinks are best at the top. That’s before considering the insidious tracking of users and the deliberate pushing of filtered stuff to them that Facebook does to make its money.
I don’t see why this is better than a forum, which has precious few of the downsides of Facebook.