Hi all.
I've run a large forum for over 20 years. I also run a company called FORUMCAST and we are an ad network that specializes in monetizing forums with CPM and CPC advertising. We primarily use Google AdX to monetize our partner forums and we are currently serving 1.3 million ads a day into our partner forums in different verticals (forum topics) to mostly US traffic
Google Adsense and Google AdX work well on forums but this depends primarily on three things:
1. traffic
2. forum subject matter (vertical)
3. geographic sources of the traffic
For the most part, the more traffic a forum gets, the more it will earn, but there are exceptions to this. We have forum partners with very low traffic but their forum verticals are in specialized subject areas. They are earning as much as large, high-traffic general topic forums. Very focused forums with technical content get very high CPM on the Google networks and especially on AdX, the largest ad marketplace in the world, because buyers are always looking to spend more to reach those markets.
Forums based in the US, UK, and Canada generally earn more because the traffic in those countries more desirable to ad buyers on the network so they pay more for it.
I was to start up a new forum today, I would look for vertical niches in the medical area, industrial components, electrical or specialized electronics, and target membership in the US and Canada.
That being said, there are things you can do to increase your revenue if you are a general topic vertical forum administrator. Things like proper ad placement, excluding certain ad sizes from serving, restricting ads from low paying areas of your forum, and other tweaks, will improve what the forum will earn. This, combined with proper targeting and selling of tyour forum to ad buyers (what we do at FORUMCAST) will maximize a forum's revenue from online advertising on the Google networks.