A record-breaking 161 million U.S. viewers watched online video in August 2009, according to data from comScore’s Video Metrix Service, which also revealed that the average viewer watched 582 minutes, or nearly 10 hours, of online video during the month.

August also saw another all-time high for the number of online videos: 25 billion were viewed during the month. Google Sites, including YouTube, accounted for more than 10 billion of these, comScore said (via MarketingVOX).

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The numbers continue to illustrate increasing growth in online video, in the number of viewers, the number of videos viewed and other measures. Figures are up significantly from July 2009, when 1.4 billion videos were viewed, with 8.9 billion of those on Google Sites. In August, comScore estimated that 81.6% of the total U.S. internet audience viewed online video. This represents an increase from 81% in July.

Google Sites surpassed the 10-billion-videos-viewed threshold, and represented 40% of all views online; YouTube.com accounted for 99% of all Google videos viewed.

More than 161 million viewers watched an average of 157 videos per viewer during the month of August, comScore said.  Google Sites attracted 121.4 million unique viewers during the month (82.8 videos per viewer), followed by Microsoft Sites with 54.9 million viewers (10 videos per viewer) and Yahoo Sites with 51.6 million viewers (6.9 videos per viewer).

Tremor Media ranked as the #1 video ad network with a potential reach of 68 million viewers, or 42.2% of the total viewing audience. YuMe Video Network ranked #2 with a potential reach of 59.1 million viewers (36.7% penetration) followed by ScanScout Network with 57.6 million viewers (35.7%).