The only forum I'm running is one I started 4 months ago, General Forums is a nice site I put up to ease my forum addiction.
The only thing holding it back is a little bit of time and funding. I'm not really running it in the traditional sense of starting with one user and one post. I've been purchasing forums and merging them into ours.
It still feels new to me, so it's a must to stop by and post at least once a day, if not more. We have some nice active members from the forums we've merged, so they've been helping with the activity as well.
The last forum I ran for 4 years before it got to the point where I could walk away from it for a week and activity would still be solid when I came back. I think this one will be like that by it's one year anniversary but I'll still be checking in every day. It's so easy with some of the mobile devices these days, it takes seconds to read and post a quick reply if needed.