I am currently a member of 6 different discussion groups- all have education and/or politics as their focus.
Those connected with an identified course tend to be rather more both facilitated and moderated than the others. Summaries etc appear that help you keep track of things and pose development questions.
Those that are interest groups have a very loose and flexible structure. I think with them the idea that you have the interest at heart i.e. "passion"- is taken to mean that you will make sense of all of that for yourself.
My son belongs to 3 discussion groups that are open and unfacilitated and he says that often he finds it very frustrating that threads seem to be left hanging.(I did ask him why he didn't do something about it, but that was not well received!!!!)
Moderation is an essential aspect of all these groups. the moderator checks posts and comments for libelous/vulgar etc contributions And in one group, requests for membership are put out to all members.