I really depends on the group's manager and the people in the group.
I have a group that is about 20K members. It is a local group and have competition with another group like it with nearly 70K members.
Facebook has reached out to me, got me on local news and invited me to a "big group" community. I beta test and all that stuff.
We (me and my team) engage with members daily and we try to correct the complaints that fans have with other groups with ours and kick out the negative stuff.
It works. It is more successful than any forum I've ran. But it doesn't make me want to give up on forums. I think forums, niche-based specifically, have more community because people did take the effort to join a website that is solely about the niche unlike that of Facebook.
I think a lot of people hate Facebook groups because they hate Facebook, which is like the Walmart of online community.