YouTube, which has had to pull copyrighted videos off its site after legal attacks by some big media franchises, has enjoyed a surge in U.S. audience share that leaves it far larger than the next 64 video-sharing sites combined, a survey found.

The U.S. market share of visits to YouTube, which Google Inc. bought for $1.65 billion last November, rose 70 percent from January through May, online audience measurement firm Hitwise Inc. said in the survey published on Wednesday.

By contrast, visits to the next 64 largest sites tracked by Hitwise rose only 8 percent during 2007's first five months.

"As of May 2007, YouTube's market share was 50 percent greater than those 64 sites combined," Hitwise research director LeeAnn Prescott said in a summary of her firm's data.

YouTube's share of the U.S. online video market was 60.2 percent in May, according to Hitwise. Its closest rival, News Corp.'s MySpace Videos site, had 16.08 percent of market share, the survey of Web surfing habits showed.

YouTube's sister site, Google Video, held 7.81 percent, while Yahoo Inc. had 2.77 percent and Microsoft Corp.'s MSN, 2.09 percent, according to the study.

Start-up Metacafe ranked No. 8 in U.S. visitors to video sites with 1.07 percent, Time Warner's AOL Media had 0.94 percent and Veoh was No. 10 at 0.86 percent, Hitwise said.

Viacom Inc. filed a copyright infringement suit against YouTube in March seeking more than $1 billion in damages and demanded that YouTube take down thousands of segments from its popular programs, including The Daily Show with John Stewart, The Colbert Report and South Park.

A separate suit was filed in early May by plaintiffs including English soccer's Premier League. Both suits argue YouTube encourages massive copyright infringement to boost the site's traffic in the hopes of generating advertising sales.

Google has responded by saying that these lawsuits threaten the way people exchange information, news, entertainment and artistic expression over the Internet.

Many of the most popular YouTube videos come from so-called user-generated sources -- the bedroom confessional produced by teenagers with cheap computer Webcams pointing at them is the archetypal format. The site's slogan is "Broadcast Yourself."

It also features unrestricted professional media programming like music videos, extreme sports feats like skateboarding, and politicians promoting their campaigns.

The Hitwise statistics track visitors to video sites, but do not capture whether or not visitors actually watched the video streams or embedded videos from these sites, she noted.

Prescott presented the data at the Searchnomics Internet industry conference held in Silicon Valley on Wednesday.