I have been using the Internet from the very beginning and various forums for almost as long. And, I have been using special interest groups even before that via things like dial-up BBSs (bulletin board systems). In the Internet era, I have used things like Usenet, BBSs, Google groups and Facebook groups. I have administrated, moderated and been a general user of quite a few over the years. Along the way, I have seen groups grow and flourish as well as decline into chaos. The key is moderation that encourages postings.
A healthy group generally has a good flow of on-topic postings. Disagreements are respectful and focused on ideas or objects rather than the deficiencies of the poster(s). These groups are driven by a core group of experts - the cadre. Any group will have posters that make comments that are wrong or off topic - the cadre deals with them gently without making it personal. Keeping that core group happy and valued is important. They make the group worth using.
Groups that have declined usually do so for one of two reasons - too much moderation or not enough.
Not enough moderation is a clear death spiral. Rude, offensive behavior becomes common and that tends to cause fewer people to post. Reduced postings causes fewer people to engage with the group - the general discord from that causes members to look elsewhere. Also, commercial interests come in and clutter the group with irrelevant postings (Make $1 Million a year with just 30 seconds of work a day!). Politics seeps into non-political groups, further alienating members. Scammers show up. The cadre goes elsewhere. Over moderation can be just as bad. It is characterized by frequent deletion of member's posts, a narrow interpretation of the groups goals or overly brittle reaction to criticism. I have been in groups where well thought out posts get deleted because there were slightly off topic. Nothing makes me leave a group quicker than that - I have better things to do than write something that will get deleted. And, I am not alone on that sentiment. Recently, I've seen threads that get locked for the simple reason that the moderator decide enough had been said on the topic. I find that arrogant to say the least. What is the harm of people adding comments? Some moderators go so far as to modify members' posts. I see this as an egregious level of control. One group run by a company associated with the open source hardware movement has even gone so far as to edit posts that are negative about the company's products or even just ask questions that expose product problems. In one case, they added a recommendation for a company product to a member's post (mine!).
So, there is "Goldilocks" moderation. Not heavy handed but still there to keep out the bad actors and irrelevant content. And, calm down the threads that are going off the rails. It keeps the core cadre happy and valued. It stops abuse of members. Good moderation makes for a healthy group.