I am currently selling my signature line on my own forum and it's earning more than Adsense. And, really that's always been true. Privately sold ads, always earn more for me than Adsense. Maybe Adsense works better on some niche forums (mine is general chat) or a niche blog. I would like to talk with you about other possible ways to monetize a forum that does not work well with Adsense. What are you doing in addition to Adsense to get earnings for your forum?
