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,