Or maybe it's exactly the opposite: the clients should have common interests (like we do here). Say 80% of them are resellers (think httpme), they will have common goals and they might be willing to try and learn from one another.

Now, if you have more clients, the forums stand a better chance to get busy. That's obvious.

However, you must build the forum, be active in it. Like Artashes did way back in 07-01-2003 when he started this very thread: create interest, ask questions, propose subjects. In a way you have to work on being a host and work on building a busy forum at the same time. You cannot put the forum there and answer only support requests, hoping the rest of it will grow by itself.