SAN FRANCISCO, US: About one of every five Internet display ads in the United States is viewed on a social networking Web site like MySpace and Facebook, according to a new report.

The report by analytics firm comScore underscores the increasing prominence of social media sites in the Internet landscape and broadening acceptance of the sites by brand advertisers.

It also illustrates the increasing competition between social media sites and established Internet companies like Yahoo Inc and Time Warner Inc's AOL which have long billed themselves as the top online destinations for brand advertisers.

The study by comScore, released on Tuesday, said social media sites represented 21.1 per cent of U.S. Internet display ads in July, with MySpace and Facebook accounting for more than 80 percent of those ads.

"Because the top social media sites can deliver high reach and frequency against target segments at a low cost, it appears that some advertisers are eager to use social networking sites as a new advertising delivery vehicle," said Jeff Hackett, senior vice president of comScore.

According to comScore, AT&T Inc, Experian Interactive and IAC/Interactive Corp's Ask Network were the top three advertisers on social networking sites in July.

While social media sites have enjoyed a surge in popularity in recent years -- Facebook is now the world's fourth-most visited Web site -- some observers have questioned whether the sites can be effectively monetized.

Because the content on social media sites is created by users, and could therefore prove racy or offensive, some have questioned the willingness of marketers to place their brands alongside that content.

"They are sensitive to some extent, but nowhere near to the extent you might think," Sanford Bernstein analyst Jeff Lindsay said of advertisers.

The price of placing ads on social networking sites is significantly less than on a Web portal like Yahoo or AOL, said Lindsay. The vast amount of Web pages available on social networks means that advertisers can purchase a massive volume of ad impressions at bargain prices.

The strategy may not be ideally suited to smaller marketers, or advertisers seeking a direct response from their ads, said Lindsay.

"For big, national brands it works just fine, just like TV," said Lindsay. "It's a huge, huge volume game."