Here are the top 15 networks for online advertising US ranked by the number of Americans it reached in the month of December 2009. The YoY growth is given within brackets
1. Aol Advertising : 187.0 million (8%)
2. Yahoo! Network : 180.9 million (8%)
3. Google Ad Network : 178.1 million (13%)
4. ValueClick Networks : 170.7 million (7%)
5. Microsoft Media Network US : 165.4 million (31%)
6. Specific Media : 165.2 million (8%)
7. FOX Audience Network : 156.9 million (N/A)
8. 24/7 Real Media : 155.8 million (9%)
9. Collective Network : 153.9 million (22%)
10. InterCLICK : 148.9 million (9%)
11. Tribal Fusion : 147.1 million (5%)
12. AudienceScience : 146.4 million (16%)
13. Traffic Marketplace : 144.1 million (-2%)
14. Adconian Media : 141.2 million (-1%)
15. Turn Inc. : 138.2 million (12%)
From: http://techcrunchies.com/largest-ad-networks-in-us/?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed:+Techcrunchies+(TechCrunchies)