A total of 171 million U.S. Internet users watched online video in January for an average of 14.5 hours per viewer, according to the latest report from comScore.

Google sites remained the top online video content property in January with 144. 1 million unique viewers, driven mainly by YouTube. VEVO landed in the second spot with 51 million viewers, trailed by Yahoo sites with 48.7 million viewers. Viacom Digital took the fourth position with 48.1 million viewers, while AOL attracted 44.5 million viewers. Google sites had the e highest number of viewing sessions with 1.9 billion, and average time spent per viewer at 283 minutes, or 4.7 hours.

Americans viewed more than 4.3 billion video ads in January, with Hulu generating the highest number of video ad impressions at nearly 1.1 billion. Tremor Media Video Network ranked second overall (and highest among video ad networks) with 503.7 million ad views, followed by ADAP.TV (432 million) and Microsoft Sites (415 million).

Time spent watching videos ads totaled 1.7 billion minutes during the month, with Hulu streaming the largest duration at 434 million minutes. 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 44.6 during the month.

Other findings from comScore in January include:

*The top video ad networks in terms of their potential reach of the total U.S. population were: Tremor Media at 46.8 percent, BrightRoll Video Network at 41.9 percent and Break Media at 40.7 percent.

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

*The duration of the average online content video was 5.0 minutes, while the average online video ad was 0.4 minutes.