From the IAB release:
The Interactive Advertising Bureau (IAB) and PricewaterhouseCoopers (PwC) today announced that Internet advertising revenues for 2006 are estimated at $16.8 billion, a 34 percent increase over the previous revenue record of $12.5 billion in 2005. The 2006 Q4 revenues totaled just under $4.8 billion, making it the highest quarter reported. Fourth-quarter revenues for 2006 represent a 32 percent increase over the same period in 2005, and a 15 percent increase over Q3 of 2006, estimated at slightly under $4.2 billion.