Internet users in the U.S. watched 12.7 billion online videos in November, an increase of 34 percent compared with a year ago, according to numbers released Monday at the market researcher ComScore.

Thanks to YouTube, Google Sites retained the crown as the top U.S. video property with nearly 5.1 billion videos viewed - or about 40 percent of all video viewed on the Internet - with video-sharing site which accounts for more than 98 percent of the traffic of Google. Fox Interactive Media was second with 439 million videos (or 3.5 per cent), followed by Viacom Digital with 325 million videos (2.6 percent).

The data also showed that 77 percent of all U.S. Internet users had viewed online video in 2008 and that the average online video viewer watched 273 minutes of video.

This is all good news for sites like YouTube and Hulu, who are trying to build a network around the video ad market. One analyst firm expects the market for video ads growing 45 percent to $ 850 million this year.

EMarketer study published in the December forecast more growth in coming years: $ 1.25 billion in 2010, $ 1.85 billion in 2011, $ 3 billion in 2012 and 4.6 billion dollars 2013.