Do you ever wonder how the various online advertising networks compare in size to one another? How much traffic each gets? Well, comScore does a monthly report, and the July numbers just came out, as part of this August 17 press release. (See table 4.) They call this report their "Ad Focus Ranking," which they define as "top entities supporting online advertising" (it's U.S. only). Note that not all sites listed in the table are ad networks, just those marked with a double asterisk.
The top five ad networks are (1) Advertising.com, (2) ValueClick, (3) Google Ad Network (yes, it's true -- they're not first in everything), (4) Tribal Fusion, and (5) Blue Lithium. The "reach" of these five sites -- meaning the percentage of the entire Internet population that visited each site at least once in July -- ranged from a high of 88% down to 66%. That represents some mighty big numbers, folks -- 158 million and 119 million uniques, respectively. These sites are the current top-dogs in the ad network world.
Of the 50 sites listed in comScore's Ad Focus Ranking, 21 of them are designated as advertising networks.