Emarketer had previously released projections on ad spending in which they reduced in this most recent projection saying, "Today’s economy, combined with the fact that social networks are still trying to come up with successful ad models, has led to lowered ad spending projections for the next few years."
Here’s their projection model for the next few years.
And here’s a great comparison showing the estimated percentage social networking ad spending will be to that of total ad spending:
Here’s a link to their article. It’s a great read!