zett - 5:02 pm on Dec 9, 2008 (gmt 0)

Another interesting data point is the amount of ads per ad block.

You see, our layouts have not changed since summer 2006. The content has been updated, but the majority of ad views are being generated by two ad sizes that can carry up to four ads. We are tracking not only the revenue, but also the number of ad blocks and the number of individual ads.

This lets you also calculate "average individual ads per ad block", just to understand how well the ad blocks are being used by Google. Are there many ads, or are many block showing just one or two ad blocks?

For us, this value has been decresing since May 2008. We have been for more than a year very close to 4 ads per ad block, which we think is a good value. Google made good use of the real estate. But since May, this value is dropping. Month after month. We're now looking at 3.5 ads per ad block on average.

My take? Advertisers are pulling out, either just out of the content network, or out of Adwords. Google does not have a full enough ad inventory that can be distributed among all publishers (especially with the number of publishers probably still rising).

Of course, one could defend Google by arguing that it may be smarter to display less ads per ad block - but our metrics (revenue, CTR, eCPM) do not support this statement. Less variety means less revenue, for us.