I think the reason why Facebook is used is due to the ease of setup and management of the group. Rather than installing forum software, paying for hosting, etc.

I think the reason why Facebook is used is due to the ease of setup and management of the group. Rather than installing forum software, paying for hosting, etc., it's just easier to use Facebook Groups. And, as adrianratnapala wrote, I think it's good to interact with those that have different values.