One can find a forum for just about any topic. I’ve been on Yahoo for several years (but note problems from previous post!). I think all of the groups that I belong to are for members only. One is for a small group of friends who live in various parts of the country and the rest are for information/help on personal interests.
Discussions forums could be a concern, but if one was set up for members only, that would keep it safe for class discussions. Yes, I do think that there could be positive aspects to using these in the classroom if set up wisely.