Social networking sites in the U.

S. accounted for more than 20 percent of all display ads viewed online in July, with MySpace and Facebook combining to serve more than 80 percent of ads, according to a new report from comScore.Over the past few years, social networking has become one of the most popular online activities, accounting for a significant portion of the time Internet users spend online and the pages they consume, said Jeff Hackett, comScore senior vice president.Social networking sites now account for one out of every five ads people view online.