My experiences with forum are both good and bad. Here I think we are talking about instructional forum, but I found some other none-instructional forums more interesting. I didn't come to any conclution why some are successful while others not, here are my cases.
I attended a training course years ago. It contained both face to face (resident)courses of 5 days each time and distance course in between. All the resident and distance courses were to be scored and were part of a criteria to grant scholarships. During the distance learning period, we use emails, not exactly forums at that time,to discuss the course which was quite active every time. The instructors observed, anwered question and joined the discussion, and at the end, we had an exam which would be used for sholarship reference. The reason why the distance course discussion are quite successful is becasue I think the discussion will also be part of the score for the scholarship which every one desired.
Some outdoor hiking websites I joined are quite active because people meet often and the forum or bbs is used as a place to share pictures and ideas. Is it part of a forum?
If forum is supported by periodic face to face activities, it seems much better interacted and insteresting. Or the topic must be very attractive with strong attention.
In CMATC's distance course as I know the forum is not so activitly participanted.
My experiences is not so positive like Vesa said, my personally like emails better.