Hi, online forums come and go.. the turn over rate is high. Visit theadminzone(dot)com it's a forum for forum admins. A while ago I started a chat forum, but it wasn't working properly, as it was really like a social networking site, but most people use facebook anyway. Although I did get hundreds of members signing up within a few months, they were not returning members.
You could try to steal members from other forums.. often people are not happy with the current forum, and are looking for alternatives.. be that special alternative and "steal" members from the boring forums.. hope this helps.
Or just study and do some
SEO, with targeted keywords for your niche. Best of luck!
Dave