The December 2009 Ad Network report was released from comScore. They provide statistics on the top 15 ad networks based on their reach. The sample size is decided from U.S. Internet users in December 2009.
You can see from the chart below that AOL Advertising had the largest reach. The reach is defined as unique visitors during the month. They also provided a percent change from the same month the previous year. The largest percentage change from the previous year was Collective Network.
See the full press release here.