BillyS - 7:36 pm on May 2, 2009 (gmt 0)

If the comparison is two years ago, we do know that Adsense is pulling in more revenue than back in 2007. So we know we have more money coming into the system.

I don't think the publisher's share has changed much in the last two years, so we know that the money flowing to publishers is higher than it was two years ago.

If everyone agrees this is true, and they also believe payouts (per click) are lower, then we can reasonalbly conclude that...

The opportunity for clicks (impressions within the system) grew at a faster pace then revenues. Therefore less revenue per click (assuming the CTR remained steady).