FT.com / Companies / Media & internet - Hefty bill for online click fraud

"Online advertisers paid more than $800m last year for fraudulent clicks on their ads and more than a quarter of them have reduced their spending as a result, according to a study by the Outsell media research firm.
The figures represent a blow to internet companies such as Google, Yahoo and Microsoft’s MSN who earn most of their revenues from advertising.
The three have avoided putting a number on the incidence of click fraud but Outsell said it averaged 14.6 per cent of all clicks billed to advertisers, even after Google and others had filtered out those ones they believed to be invalid.
The 14.6 per cent equates to $800m of the $5.5bn US search engine market in 2005."