Together, Google AdSense and Google-owned DoubleClick account for 57% of total ad server market share, while Microsoft and Yahoo lost significant share since March and together make up only 15% of the ad server market, according to an analysis by Attributor.

The increase in new ad networks in 2008 had a slight impact on the market, but only one newcomer — Revenue Science — broke the top five.

Google's AdSense is dominant on smaller sites while DoubleClick leads on larger sites. AdSense, however, made great strides on larger sites, leapfrogging Yahoo for second place since March.

Additional findings from the analysis:

• DoubleClick and AdSense continue to dominate ad server market share, capturing 31% and 26% of unique users, respectively.
• For larger sites with more than 1M monthly unique users, DoubleClick has a 36% share, a three-times share advantage over its nearest non-Google competitor, Yahoo. Together, AdSense and DoubleClick capture 54% of this segment.
• For smaller sites with less than 1M monthly unique users, AdSense has a 38% share, a more than a seven-times share advantage over its nearest competitor, ValueClick. Together, Google and DoubleClick capture 60% of this segment.
• AdSense gained two points of share on large sites while Yahoo lost four points since the March report.
• Share positions for the 'Big 3' — DoubleClick, AdSense and Yahoo — vary widely by vertical.
• DoubleClick share is strongest on Automotive sites (58%), whereas AdSense is strongest on Blog sites (40%). Yahoo is strongest on Health sites (13%).

About the research: Attributor analyzed the ad server calls across 75 million domains as part of its October 2008 crawling operations. The data was then combined with compete.com's October 2008 traffic data and its vertical site category classification to provide the unique user and content category breakdowns.

Each share total represents the sum of all ad networks owned by each company, with Google as the exception in which DoubleClick and AdSense are displayed separately. For example, Atlas DMT share is counted within Microsoft share numbers and Advertising.com is included in AOL share numbers. On 12/19/08 Attributor determined it was not detecting the share for OpenX correctly and is working to fix the problem for its next release.