A forthcoming report by IDC estimates that online advertising in the U.S. reached $25.5 billion in 2007, and $7.3 billion in the fourth quarter.

It also puts Google’s market share of Internet advertising in the U.S. during the fourth quarter of 2007 at 23.7 percent, down half a percentage point from the third quarter. That is Google’s first slip in market share in two years. While Google’s overall U.S. sales (net of traffic acquisition costs that goes to pay partner sites) still went up 40 percent last quarter, it was not enough to keep its market share position compared to the overall industry. Damn those non-performing MySpace ads!