zdgn - 1:59 pm on May 2, 2009 (gmt 0)
I also think it's a growth mismatch. A case of over-saturation of content. Most veteran sites had unique -- for lack of a better term -- content or first-mover-advantage in their relevant Adsense publisher marketplace, so they did well in the beginning. The exponential growth of me-too publishers whose only purpose is to infiltrate niches (without much qualification/experience to be in that niche) in order to get a piece of the content pie is one of the causes of more takers and fewer givers (not that it's necessarily a bad thing, it's a free market, right?) and hence, The Incredible Shrinking AdSense Pie. So, I think the growth of content servers (regardless of quality) hasn't matched the growth of advertisers.