Social media sites are gaining acceptance of brand advertisers in the Internet landscape. About one out of five online display ads in U.S. is viewed on a social networking site, according to a new report by comScore, Reuters reported.

It also pointed out the intense competition between social media sites and big online companies, including Yahoo! and AOL, which have long positioned themselves as the leading Web destinations for brand advertisers.

Social media sites contributed 21.1 percent of U.S. Internet display ads in July, with more than 80 percent coming from MySpace and Facebook, according to the study released on Tuesday.

"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," Jeff Hackett, senior vice president of comScore, said, Reuters reported.

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

Though social media sites have boosted in popularity recently - Facebook is the world's fourth-most visited site now, some still questioned if the sites can be effectively monetised.

Due to the fact that the user-generated content could be "racy or offensive", some have doubts if marketers would like to place their brands alongside that content.

"Advertisers are sensitive to some extent, but nowhere near to the extent you might think," according to Sanford Bernstein analyst Jeff Lindsay, Reuters reported.

Placing ads on social networking sites is significantly cheaper than that on a Web portal. "The vast amount of Web pages available on social networks means that advertisers can purchase a massive volume of ad impressions at bargain prices," said Lindsay.

Although the strategy may not fit smaller marketers, or those who want a direct response, "for big, national brands it works just fine, just like TV," Lindsay added.