The digital market analyis firm comScore counted 1.1 trillion online display ad impressions delivered to U.S. viewers in the first quarter of 2011. The top five advertisers online during this period were AT&T, Experian, Scottrade, Intuit and Verizon. Social buying network Groupon was also high on the advertiser list.

Of that 1.1 trillion total, comScore reports Facebook at the top of the list, serving 346 billion impressions (representing 31 percent of the total). Facebook nearly doubled its share of the market from the same period in 2010. Yahoo sites came in second, serving 112 billion impressions (10 percent of the total impressions for the quarter).

What comScore doesn't break out in this report are rankings based on spending. Based on the limited spending numbers they do provide, we guess that the average spending rate works out to around a nickle for 100 impressions. Not surprising given the enormous volume of relatively low-cost Facebook impressions.

Read the comScore press release here.