I see this all the time. People come on admin forums and ask why their new forum is dying/not getting more active/not doing so well. Everyone then pretty much says the usual stuff about how forums are on the way out or how it's just bad luck, you know the drill.
But then I actually look at their forum and notice something.
They don't even post on their own site regularly.
Maybe at best they've posted once or twice in the last 24 hours (usually just replying to topics), maybe they've literally not done anything at all for the last day/week/month and hoped their magical users would save their forum and post hundreds of posts a day without any admin intervention.
How the hell do they expect their forum to keep doing well then? Forums aren't easy, you don't just set up some content on day 1 then sit around for the few months while your forums are magically filled with new content on a daily basis.
It might sometimes seem like that initially. Maybe you've got a popular blog or wiki, or are a Youtube celebrity, or actually famous offline. But it won't last. No one is 'popular' enough that people can keep their site going for them while they sit back and do nothing, and I've seen tons and tons of popular sites and people open forums whch have failed in mere weeks.
So here's some very simple, extremely obvious advice for forum management. Post constantly. Posts news and articles you find interesting. Write your own articles on at least a weekly basis and start your own topics in your forum for members to reply to. You won't get anywhere doing nothing at all, and the world doesn't owe you living, so keep your site interesting with regularly, hopefully at least daily content.