According to comScore, 1.1 trillion ads were served to U.S. internet users in the first quarter of 2011. Of those 1.1 trillion ads, 346 billion of those (31%) were on Facebook. That's right -- you read that correctly. Facebook now serves nearly one third of all online advertisements in the U.S.
To help put this data into perspective, consider the representation of some of the other major online companies. Yahoo has a 10 percent share, Microsoft has 5 percent, and Google holds 2.5 percent. Basically, no other online company comes close to Facebook's market share.