The Interactive Advertising Bureau reports that Internet advertising exceeded $4.2 billion for the third quarter of 2006, a 33% increase over the same period last year. 
This chart from PWC/IAB shows the rapid growth in on-line advertising. Remember, this is revenue per quarter, not cumulative revenue over time.
Google recorded Q3 revenues of $2.7B, accounting for 64% of all on-line advertising.
Online advertising is big business and it is growing very fast as advertisers move more of their budgets from traditional advertising (print, radio, TV) to more interactive on-line advertising. Internet advertising still accounts for less than 10% of overall ad spending.
No trend continues on a straight line forever. There will be ups and downs, new advertising markets, and new competitors. But, this looks like a very solid business with good growth prospects over the next several years.
