Online video continues to gain in popularity and 161 million U.S. Internet users watched online video during August, the largest audience ever recorded, according to comScore.

In August more than 25 billion videos were viewed, with Google sites accounting for more than 10 billion.

Google sites remained as the top U.S. video property in August as it surpassed the 10 billion videos viewed threshold, representing 40 percent of all videos viewed online. YouTube accounted for 99 percent of all videos viewed on Google sites.

Microsoft sites ranked second with 547 million (2.2%) followed by Viacom Digital with 539 million videos viewed (2.1%) and Hulu with 488 million (1.9%).

More than 161 million viewers watched an average of 157 million videos per viewer during the month of August. Google sites pulled in 121.4 million unique viewers during the month (82.8 videos per viewer) followed by Microsoft sites with 54.9 million viewers (10 videos per viewer) and Yahoo sites with 51. 6 million viewers (6.9 videos per viewer).

Tremor Media ranked as the top video ad network during the month with a potenial reach of 68 million viewers, or 42.2 percent of the total viewing audience. YuMe Video Netowrk ranked second with a potential reach of 59.1 million viewers (36.7 % reach) followed by ScanScout Network with 57. 6 million viewers (35.7%).

Other highlights from the report include:

The top video ad networks in terms of their actual delivered reach were: Tremor Media Video Network (19.5 percent penetration of all online video viewers), BrightRoll Video Network (16.2 percent penetration), and BroadbandEnterprises.com (14.8 percent penetration).

• 81.6 percent of the total U.S. Internet audience viewed online video.

• The average online video viewer watched 582 minutes of video, or 9.7 hours.

• 120.5 million viewers watched nearly 10 billion videos on YouTube.com (82.6 videos per viewer).

• 44.9 million viewers watched 340 million videos on MySpace.com (7.6 videos per viewer).

• The average Hulu viewer watched 12.7 videos, totaling 1 hour and 17 minutes of videos per viewer.

• The duration of the average online video was 3.7 minutes.