If everyone is earning so little on Revenue Share at DP (which is a large traffic site), think about all those smaller revenue sharing sites....
Usually those revenue sharing sites offer it as an.. hmm say incentives..appreciation that you dedicate your time to the site. But can't be taken as any real serious source of income. Besides, the fact that forums have a terrible ppc rates I can imagine, and a very low click thru rate. Just imagine it, the biggest forum for webmasters, website owners.. most of us are webmasters and will rarely click on the AdSense ads cause we know them. The serious revenue comes from the end-user, or whatever you call it.. targeted visitor.