I run a 120x600 Adsense skyscraper ad that is not performing well. I want to replace this adsense skyscraper ad with skyscraper CPM banner. Anyone know good CPM network that accept forum site with skyscaper ad?
Forum sites generally perform quite poorly in advertising networks; I doubt you will earn more by switching to other networks, although you might benefit from alternating ads from more than one ad network (Google AdSense, Yahoo Publisher Network, AdBrite, Bidvertiser, etc.)
The problem with forum sites is that your visitors are focused on a specific task: reading the messages, and posting messages. And your site probably has a fixed layout so the ads are in the same place all the time, so your visitors will "tune them out." (Juggling your page so that the ads are more noticeable will just annoy your visitors.)
I am not aware of ANY CPM networks that can generate reasonable revenue for forum sites; Google AdSense is quite probably the best-performing placement you're going to find, although as noted above, you might benefit from rotating other ads from other sources.