Google Inc.'s (GOOG) sites continued to dominate online-video viewing statistics in August, as the company surpassed the 10 billion mark and took in 40% of all viewership, according to comScore Inc.

The search-engine giant's popular video site YouTube was the main source of the traffic, accounting for 99% of all videos viewed through Google's sites. Google has been under pressure to boost revenue from YouTube, which the company bought several years ago for $1.65 billion. The online-video advertising market is seen as a key source of new revenue for Google.

Microsoft Corp.'s (MSFT) sites ranked a distant second with 547 million viewers, or 2.2% market share. Microsoft was followed by Viacom Inc. (VIAB) with 539 million videos viewed, or 2.1%, and Hulu with 488 million, or 1.9%. Hulu is a joint venture of General Electric Co.'s (GE) NBC Universal and News Corp. (NWS), which owns Dow Jones, the publisher of this newswire.

Video viewing reached an all-time high in August with more than 25 billion watched. Google's sites attracted 121.4 million unique viewers during the month, watching an average of 82.8 videos, followed by Microsoft sites with 54.9 million viewers and 10 videos per viewer and Yahoo Inc.'s (YHOO) sites with 51.6 million, or 6.9 videos per viewer.

The average online visitor watched 9.7 hours of video during the month, according to comScore. The duration of the average online video was 3.7 minutes.

-By John Kell, Dow Jones Newswires; 212-416-2480; john.kell@dowjones.com