A report by research firm comScore concluded that about one in every five ads on the Web is viewed on a social networking site. According to the study, social media sites accounted for 21.1% of US Internet ads during June -- heavy hitters MySpace and Facebook represented about 80% of those ads. The analysis highlights the growth of social media online, as well as the increasing interest advertisers have in social sites.

With a surging appeal in online social networking over the last few years, the sites are able to deliver high reach and frequency against target segments at a low cost. Facebook is now the world's fourth-most visited Web site and had nearly 27 billion display ad impressions in June. MySpace topped that with over 30 billion impressions. In a distant third place, Tagged had almost 2 billion impressions, and MocoSpace came in at just under 500 million.

AT&T reportedly ranked as the top display advertiser on social networking sites with over two billion ad impressions. Advertisements on social sites accounted for 30% of the carrier's entire arsenal of display ads in June. Experian Interactive scored second with almost 1.3 billion impressions, Ask Network had 950 million, Sprint Nextel 790 million, and Pangea Media 572 million.