I think finding a solution to the problem of sustainability on small forums could start to be found by considering discussion as a low population or team requiring activity and then taking inspiration from the others. Examples being minigames in MMORPGs, dancing, sports, or clubs instead of just looking towards web social platform development. I think one of the problems shared is asynchrony. One of the ways they solve the asynchronous hurdle is by co-scheduling events with each other.

For example, it wouldn't be fulfilling to go a tennis court expecting a partner and them to only maybe to show up two weeks after you expected them to. By the time they show up you might no longer be in the mood, and so they themselves may expect this and not attempt to show up late even at all. Discussion is clearly more elastic in the required synchrony of the participants but I believe there is a limit – which small forums run into long term to the detriment of them.

Following off of the above, for a variety 'freeform' forum it could maybe be of beneficial effect to have organized spotlights weeks or by batching post releases. This would be used for slower topics, or the topics people think less of and feel more through in their experience of them.

You may not be unified around a single common interest but your trajectory and experience through topics could (partially) be.

Some predictability for when posts and returning discussion should arise on topics would probably be fulfilling to readers as well as posters looking for discussion. Beyond just making participation more satisfying it increases memorability of the forum by charting itself precisely into a person future as a time to check back. You could even seed a new spotlight session with popular posts from past sessions as a way of introducing new participants to old community hallmark posts as well as guaranteeing something to be there to see when you show up.

If you used spotlight weeks for certain topics, it also helps be a middle ground solution between having a broad topic board that has no direction and siloing a subtopic into its own isolated board before it has the ability to actually self sustain itself – which is another issue I think small forums run into long term.

FYI 1000 hours, is about 41 24-hour days. Assuming you play a game for only 4 hours per day, that's a little over 166 days. I think everyone one of us has played something longer than 166 days (especially if you were one of THOSE MMO players back in the day), so it doesn't hold water to me.