CTR Forums are generally considered low CTR performers, with many members on this forum reporting on the order of under 1% CTR. That's in general of course.
Good, Bad and Ugly of User Generated Content The AdSense success of a forum depends not only on the niche and buy cycle quality of the traffic (like any other site), but it also has to jump the hurdle of how well post titles are optimized. It's User Generated Content, which works great for longtail success and jumping into the vanguard of sites ranking for trending topics. The success is often hindered/balanced by a surfeit of posts titled, "Help!" So an action plan of moderation must be in place if you wish for your community to perform optimally.
Here is a helpful and inspirational post written by someone who experienced wild success with a social networking site: