Social Networks Ad Spending

E-Marketer changed its prevision for ad spending on social networks in 2007 (from $865 to $900 million).
Should we just joy or should we start to fear a new crackdown?

MySpace and Facebook account for 72% of the revenues...
No wonder that new social networks are appearing every day and that existing websites start to add social network capabilities

Here a list of the most popular ones, ranked per Market share of visits (click to enlarge)


And here the article: The crowded world of social network