Here's a good statistic to start with from a recent home page topic...
Americans watched more than 9 billion videos online in July, up about 8 percent from the month earlier, according to comScore Media Metrix. The bulk of the videos were viewed on social networking sites. Google's YouTube ranked at the top with about 2.4 billion videos views. Yahoo, in second, had 390 million videos viewed and Fox Interactive Media, which owns MySpace, came in third, with 298 million views for July.
9 Billion Videos in 2007 July! 9 Billion! That's a lot of videos. And YouTube served 2.4 billion of those.
Here's the comScore Press Release
U.S. Viewers Watched an Average of 3 Hours of Online Video in July [comscore.com...]
Nearly 75 percent of U.S. Internet users watched an average of three hours of online video during the month. Google Sites topped the July rankings with both the most unique video viewers and most videos viewed.