Top 15 U.S. Ad Networks by Unique Visitor, February 2009

Platform-A** 172618
Yahoo network** 163406
ValueClick Networks** 157753
Google Ad Network** 156481
Yahoo sites 144304
Specific Media** 142774
Google 140863
Traffic Marketplace** 139372
Tribal Fusion** 137876
24/7 Real Media** 137409
Tremor Media 132219
Casale Media-MediaNet** 131982
interCLICK*** 125403
Adconian Media Group 124792
Microsoft Media Network U.S.** 123363

Source:http://www.clickz.com/3633207