I used to be involved in a very successful one, but it seems to have been ruined by the increasingly-heated discussion of "politics" (a word that growing numbers of people are bizarrely using to refer to dumb "culture-war" issues) beginning in 2016. There are too many people who will find any excuse to bring discussion around to their vapid ideologies, which led to a ban on the discussion of anything "political" on the board.
Unfortunately, it turns out that a rule like actually ends up stifling a surprising amount of otherwise fruitful discussion, including much that you would never have thought of as violating such a rule until you see it start being enforced.
As for when the group was successful, what I would advise is:
• Make it about something that people could actually have a lot to say about. This is really the most important thing, the rest will probably follow naturally as long as the admin team isn't distressingly incompetent.
• Have a team of admins with slightly differing perspectives on the rules to avoid groupthink. All of them should also have well above average intelligence, and should probably be older than thirty or so.
• Make it a closed group, but one to which access is pretty much a rubber-stamp procedure as long as the person's profile doesn't seem to obviously be a bot or troll.
• The ethos of the board ought to be one of complete amorality. If discussions seem to be veering into talk of "right and wrong" too often, this is a good sign that there is probably little going on in your group any longer which is actually worthy of discussion.
• Low tolerance for inane nonsense posts, meme-spamming, etc. People who are mostly making these kinds of posts should be summarily banned.
