Don't look now, but Facebook is rapidly emerging as a force in online video, according to comScore. In May, Facebook.com ranked fourth among all online sites with 48.2 million video viewers.
While the top social network does host user-uploaded videos, it mostly aggregates the YouTube clips that users publish on their walls. That still helped it beat out Viacom Digital, which ranked fifth in May with 46.5 million online video viewers.
Google Sites, driven primarily by YouTube, ranked as the top online video content property in May with 147.2 million unique viewers, followed by Vevo with 60.4 million viewers and Yahoo Sites with 55.5 million viewers.
Overall, 176 million U.S. Internet users watched online video content in May -- an average of 15.9 hours per viewer. What's more, the total U.S. Internet audience engaged in more than 5.6 billion viewing sessions during the course of the month.
Google sites had the highest number of viewing sessions with more than 2.1 billion and the highest time spent per viewer at 311 minutes, crossing the five-hour mark for the first time, comScore found.
Americans viewed 4.6 billion video ads in May, with Hulu generating the largest number of video ad impressions at more than 1.3 billion.
Tremor Media Video Network ranked second overall -- and highest among video ad networks -- with 700.8 million ad views, followed by Adap.tv -- 642 million -- and BrightRoll Video Network -- 565 million.
Time spent watching videos ads totaled more than 2.0 billion minutes during the month, with Hulu delivering the highest duration of video ads at 560 million minutes.
Video ads reached 45% of the total U.S. population an average of 34 times during the month. Hulu also delivered the highest frequency of video ads to its viewers with an average of 48 over the course of the month.
Also worth noting, the top video ad networks in terms of their potential reach of the total U.S. population were: Tremor Media at 47.1%; BrightRoll Video Network at 42.1%; and Break Media at 40.4 percent. Overall, 83.3% of the U.S. Internet audience viewed online video in May, while the duration of the average online content video was 5.2 minutes, and the average online video ad was 0.4 minutes.
Video ads accounted for 12.6% of all videos viewed, and 1.2% of all minutes spent viewing video online, comScore found.