Based on eMarketer’s data, 70 percent of all U.S. teens currently visit social network sites on a monthly basis.

“By 2011, one-half of all online adults and 84 percent of online teens in the US will use social networking each month. There is little to suggest that this activity will go away,” adds Williamson.

In terms of advertising revenues of this market, eMarketer projects that global online social network ad spending will grow from US$1.2 billion this year to $2.2 billion in 2008, and exceed US$4.1 million in 2011.

The U.S. spending is projected to increase from $920 million in 2007 to $1.6 billion in 2008.

”MySpace and Facebook together receive more than 70 percent of all U.S. social network ad spending,” according to Williamson. “And they are hard at work to convince marketers to allot more of their budgets to social network advertising.”

The advertising offerings of the two leading social network sites are becoming more diversified, and now have search, display ads, widgets and more, not just merely profile pages.

“But if social network marketing delivers on its promise of peer recommendations the flow of advertising dollars will turn into a flood,” says Ms. Williamson.