U.S. online users watched 12 billion online videos in May, up 45 percent from the same period in 2007 and nearly 10 percent from April, according to the latest data from comScore's Video Metrix service.

Google Inc., which owns leading video sharing site YouTube, again topped the chart with a 35 percent share of all videos viewed. Fox Interactive Media followed far behind, with 6.4 percent market share, which, however, was up 1.3 percent over last month, Media Post reported.

Others on the top five list were Yahoo (2.9 percent), Microsoft (2 percent) and Viacom Digital (1.7 percent), Media Post reported.

Hulu.com, the joint venture of NBC and Fox featuring full episodes of their TV shows, debuted on the chart at tenth place, with a 0.7 percent share.

In terms of unique viewers, the race was much more intense. Google led again with almost 84 million unique viewers, while Fox Interactive was in second with 60.7 million. Yahoo, Microsoft and Time Warner (excluding AOL) followed with 40 million, 29.5 million, and 24.6 million, respectively, according Media Post.