Forums are known as poor performers when it comes to Google Adsense. Most forum users are ad blind and even if they see the ads, they tend to never click. I use to say that forum users are the worst type of users because they consume the most resource and give nothing back.
Having said there, I would like to add that it is possible to make good money off Google from a forum. You just have to be willing to get a little more “in your face” with your forum population. I came across an amazing vBulletin Google Adsense Deep Integration Tutorial at over at vBulletin.org.
This tutorial is written to show you how to use your Google Adsense codes efficiently in your vBulletin (3.5+) templates. I would use it on my forums but I’m still using an older version of VB at this time. I do plan to update and use the advice in the tutorial however.