U.S. social networking sites still rank among the most popular online destinations, especially among younger audiences, according to eMarketer.

Leading players MySpace and Facebook combined will make up 72 percent of the total U.S. social network ad spending, or more than US$ 1 billion, this year, according to AdWeek.

Spending at other online social network sites is expected to total $370 million this year, eMarketer estimated.

In addition, widget and application ad spending will boost from $15 million in 2007 to $40 million this year, AdWeek reported.