comScore said today that 178 million U.S. Internet users watched online video in August for an average of 14.3 hours per viewer.

Google sites, driven by video viewing on YouTube, ranked as the top online video property with 146.3 million unique viewers. Facebook move up one position to land in the second spot with 58.6 million viewers, for a total of 243 million viewing sessions. Yahoo sites ranked third with 53.9 million viewers, followed by Vevo with 45.4 million. Google sites had the highest number of overall viewing sessions with 1.9 billion and average time spent per viewer at 270 minutes , or 4.5 hours.

Americans viewed more than 3.8 billion video ads in August, with Hulu generating the highest number of video ad impressions at 790 million. BrightRoll Video Network ranked second overall (and highest among video ad networks) with 469 million ad views, followed by Tremor Media Video Network (442 million) and Microsoft Sites (234 million). Video ads reached 45 percent of the total U.S. population an average of 28 times during the month. Hulu delivered the highest frequency of video ads to its viewers with an average of 30 over the course of the month.

Other highlights from August include:

*85.1 percent of the total U.S. Internet audience viewed online video.

*The duration of the average online content video was 4.8 minutes, while the average online video ad was 0.4 minutes.

*Video ads accounted for 10.7 percent of all videos viewed and 1.0 percent of all minutes spent viewing video online.