In my experience the Facebook groups always turn to crap, especially if it's a group that attracts more than about 500 users. Abusive posts, scam posts, fake groups with the same name created by bots. I've reverted to old school forums for all my special interests. Marketplace is still the best classifieds product though.

It depends on the mods and the specific communities the groups are about. I have seen what you describe in some, but not at all in others.