comScore Video Metrix has released its September online video rankings finding that 175 million U.S. Internet viewers watched online video during the month for an average of 14.4 hours.

Google sites, driven mainly by video viewing at YouTube, continued to rank as the online video content property with 52.2 million unique viewers. Yahoo sites move back into the second spot with 54.4 million viewers after falling to Facebook in Augst. Facebook closely trailed with 52.2 million viewers. Microsoft moved up 3 postions in September, landing in fourth place with 45.5 million viewers. Google Sites had the highest number of overall viewing sessions with 1.9 billion and average time spent per viewer at 260 minutes, or 4.3 hours.

Americans viewed more than 4.3 billion video ads in September, with Hulu generating the highest number of video ad impressions at 794 million. Tremor Media Video Network ranked second overall (and highest among video ad networks) with 526 million ad views, followed by BrightRoll Video Network (476 million) and ADAP.TV (444 million). Video ads reached 45 percent of the total U.S. population an average of 32 times during the month. Hulu delivered the highest frequency of video ads to its viewers with an average of 27.5 over the course of the month.

Other highlights from comScore include:

*The top video ad networks in terms of their potential reach of the total U.S. population were: ScanScout at 46.8 percent, BrightRoll Video Network at 44.5 percent, and Break Media at 44.1 percent.

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

*The duration of the average online content video was 4.9 minutes, while the average online video ad was 0.4 minutes.

*Video ads accounted for 12.3 percent of all videos viewed and 1.2 percent of all minutes spent viewing video online