YouTube's U.S. audience grew significantly in the first six months of 2006 - it was the fastest growing site among the top 25 web brands in terms of unique audience, according to Nielsen/NetRatings.

In the first six months of the year, YouTube's unique U.S. audience increased 297 percent, reports Nielsen/NetRatings (pdf): from a monthly unique audience of 4.9 million to 19.6 million - a fourfold increase. But the number of pageviews has grown even more: 515 percent - from 117.6 million in January to 724.0 million in June, or a sixfold increase. Moreover, the average time spent at the site increased 64 percent, from just over 17 minutes into nearly 28 minutes.

And in one week in July - the week ended July 16 - the number of unique visitors to YouTube increased 75 percent: from 7.3 million in the previous week to 12.8 million unique visitors. (Likely not coincidentally, that was the week a video of the infamous Zidane headbutt at the World Cup was posted to the site, and linked to from the New York Times; the video has been viewed nearly 2.65 million times.)

Men are 20 percent more likely to visit YouTube than women, with unique audience composition indexes of 113 and 88, respectively (the average is 100). Visitors age 12-17 (compostion index, 142) are nearly 1.5 times more likely than the average web user to visit YouTube.