It's official: Online video advertising is a force to be reckoned with. More than 11 billion ads streamed over the Web in June, amounting to 25% of all video content viewed.

That online video ad traffic, as reported by comScore, translated to 4.6 billion minutes watched by 180 million Internet users in the United States. The average duration of an online ad was four seconds.

The top five ad networks streamed at least one billion ads each. Google took the lead with 1.41 billion ads streamed. Google's domination isn't surprising; YouTube serves more than 99% of video on Google-related properties and is the third most popular website in the world, with 72 hours of video content uploaded every minute.

BrightRoll, a San Francisco-based video network serving thousands of sites including Google, came in a close second with 1.39 billion ads served.  BrightRoll bills itself as "the world's largest and fastest growing advertising network." It announced in an official blog post that the company now holds the number-one position for reaching video ad viewers in terms of duration. (BrightRoll watchers viewed 805 million minutes of ads.) 

Hulu came in third in video ad rankings, but it delivers the "highest frequency of video ads to its viewers with an average of 52" ads per person.

Greg Jarbone at Search Engine Watch theorized that Hulu's drop in ad sales to third place could be partially related to "the lack of hit shows on TV," a.k.a. the summer TV season lull. Jardone's theory has merit. The most popular (and new) video shows on Hulu over the past month have been Gordon Ramsey's reality television projects, "Hell's Kitchen" and "Master Chef." 

The record-breaking number of ads streamed comes on the heels of a New York Times article pointing out TV watching among young adults is at a low, or as the headline put it, "The Case of the Disappearing Viewers." (Mystery solved: Young folks watch TV on the Web.)  Incidentally, the TV shows that experienced their lowest audience numbers, according to the New York Tmes, were the most popular shows on Hulu in the spring. These included the NBC Thursday night lineup ("Community," "Parks and Recreation," "The Office," and "30 Rock"), Fox's "Glee," and ABC's "Modern Family." The coveted young-adult audience has moved online, and Internet video ad networks are capitalizing on that shift. 

Image courtesy of comScore.