U.S. advertisers will spend $1.68 billion on social networking sites such as Facebook, MySpace and Twitter in 2010, according to a new report from eMarketer.

The $1.68 billion figure represents a 20 percent increase over 2009 on social network advertising spending and is set to climb to more than  $2 billion in 2011.

In December 2009, eMarketer estimated $1.3 billion in social network ad spending in 2010. Healthy online ad spending in general, and Facebook in particular, has resulted in the increased forecast.

Facebook will pull in half of all social network ad spending in the U.S. while "MySpace continues to diminish in importance." Spending on Twitter will be low in 2010 but the "potential for 2011 and beyond could be dramatic."

Spending on social network advertising will grow even more quickly globally. eMarketer forecasts just over half of social network ad spending globally will come from the U.S., but 2011 will see a reversal.

Social gaming companies such as Zynga and Playdom are expected to attract $293 million in spending worldwide in 2011, up from $220 million in 2010.