Google will end 2005 with $6.1 billion in revenue. About 99 percent of that revenue comes from keyword ads (over 56 percent from AdWords, according to the company's most recent quarterly financial statement, and 43 percent from AdSense), making Google a bigger recipient of ad dollars than any television network or newspaper chain. All of which is to say that little blue text links, a type of advertising that barely existed five years ago, are poised to become the single most important form of marketing in the US - unless click fraud ruins it.
But some observers, like Holcomb, believe that click fraud is "a billion-dollar mess" that "has the potential of destroying the entire industry."
http://www.wired.com/wired/archive/14.01/fraud.html