Summary:

Hulu topped all other ad networks for online video ads served, with 1.3 billion ad impressions during the month of May, according to comScore. That’s more than a quarter of the 4.6 billion ads that were served up by online video properties during the month.

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Updated. Hulu topped all other ad networks for online video ads served during the month of May, with 1.3 billion ad impressions, according to comScore’s latest Video Metrix report. That’s more than a quarter of the 4.6 billion total ads that comScore reported were served up by online video properties during the month.

What’s the secret to Hulu’s success? It typically serves four or five times as many ads to consumers as comparable ad networks or networks sites. On average, Hulu viewers were served 47.6 ads in the month of May. Compare that to Viacom, with 11.7 ads per viewer on average, or CBS, with 11.3 ads per viewer. The only other network site that even comes close in comScore’s rankings is ABC, which displayed 22.7 ads per viewer.

Of course, part of the high ads per viewer rate is also due to the huge amount of time that viewers spend on Hulu. YouTube is still king in terms of online viewers and videos served, with more than 2.1 billion videos served and 311 minutes (or about five hours) of video consumed per viewer in May. But Hulu isn’t that far behind, at least in terms of time spent on site: its viewers each watched an average of 218 minutes of video — or about three and a half hours — during the month of May.

While Hulu still lags behind YouTube in terms of videos served, it has established itself as a leader in monetizing the videos that viewers do watch. Last year, it reported $260 million in revenues, the vast majority of which were driven by ad-supported viewing. The startup expects to grow that revenue to more than revenue to approach $500 million this year, driven in part by increased viewership, and also by growth in its Hulu Plus subscription membership.

Ed.: Kilar’s blog post said revenues would approach $500 million in 2011, not exceed that amount.

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