Hi all.... My 10p's worth.
I used to be an admin on cable forum and we had loads of active members. I think the key is to keep people on all the time so they can reply to threads fast. The key to this I believe is to have a social area (we called it the basement at CF) so that users can discuss whatever they want, News entertainment that sort of thing. Online games ect can allso keep people interested.
Whats good about this site it that it is technicaly focused but thats allso its downside as users only come on mostly to answer or ask a question. If users had other areas to live in that would keep the activity up.