This is an open group that you're participating in now. Anyone can join. Also, everyone on the Internet can see anything you post here, they just have to create an account to post anything themselves. And it's anonymous, more so than most facebook activity. The only thing you see in here is pseudonyms people are making up, compared to facebook, where the majority (not all) people tend to use their real names to communicate with their friends/family. A closed group in facebook means no one else outside the group can even see the posts.
People in closed facebook groups can still see other group members, just like you can see other forum members in here. You can also view their facebook pages, if their facebook security configuration permit it. Most facebook users know very little about security, and more often than not they don't turn much of it on at all. So in most ways, facebook is actually less private, even with closed groups, than forums like this.
I have a friend that participated in a facebook group and ended up getting stalked, initially through facebook. The creep that stalked her managed to find out a whole lot about her private life, because he just looked at her page, and pages of other people that were posting to her page. Didn't take him long to find out where she lived, worked, pics of her and her family, her email address, phone number, and more. And all because he didn't like her posts. That was a real nightmare for her.