Four years in, around 4,000 registered users, and I’m still the primary driver of forum activity. The forum averages somewhere between 5 and 15 new topics a week and around 65 active users (including my activity which varies ~ 20 to 30 percent.
The core challenge is that most active members have to be there because they genuinely want to be. There’s no external pull, no support requests tied to a product or service bringing people through the door to solve issues, no built-in reason to return.
Finding and retaining people who show up purely to participate with peers of a niche interest is a different problem from running a product-backed community, and it’s one I haven’t fully cracked yet. 
I’ve tried things like daily discussion topics to keep things moving, which helps at the margins, but the community still hasn’t reached the point where it sustains itself without my large involvement.
I’m curious whether others here are running general interest communities in a similar position and what’s actually moved the needle in terms of getting members to show up and engage on their own. To the point where activity is healthy and trending upwards even without babysitting it, as it were.