Botnets are driving up click fraud, according to Click Forensics, a click auditing company.

"This is a growing problem for the industry and one that's getting worse and not better," said Tom Cuthbert, president and CEO of Click Forensics.

The company's findings are based on data from its Click Fraud Network, which attempts to identify and track pay-per-click advertising fraud across over 4,000 online advertisers and ad agencies.

Click Forensics claims that the overall industry average click fraud rate during the second quarter of the year was 15.8%, up from 14.8% in the previous quarter and from 14.1% during the second quarter of 2006. Traffic from botnets -- coordinated groups of computers that have been subverted by an attacker -- doubled from first quarter to the second quarter this year, the company says.

Last month, the FBI said it had identified about 1 million compromised computers, or bots, in the United States.

Click fraud on content publishing networks, such as Google AdSense and Yahoo Publisher Network, jumped from 21.9% in the first quarter to 25.6% in the second, according to Click Forensics.

Content publishing networks tend to exhibit higher rates of questionable clicks because publishers have a financial incentive to participate in click fraud. Of course, everyone involved has a financial incentive: Click Forensics' business depends on the fear of click fraud.

Google also benefits from click fraud because it sells clicks. But it fights click fraud all the same because customer trust is worth more.

On Wednesday, a member of Google's AdWords team posted an update on Google's efforts to fight click fraud. Google has recently introduced a way for advertisers to keep their ads off of certain sites. It has made an effort to be more transparent in how it deals with click fraud. The company has even published research on click fraud botnets.

Back in January, Google's Shuman Ghosemajumder, business product manager for trust and safety at Google, accused Click Forensics of "basic counting mistakes and inflating the number of clicks by an average of 40%."

But Cuthbert don't accept this. "We're confident in our methodology," he said.

Google's position remains the same. "These estimates continue to count clicks Google does not charge to advertisers as fraudulent, so they are not actually click fraud estimates," said Google spokesperson Barry Schnitt in an e-mail. "Furthermore, their estimates have never reflected the invalid click rates we see at Google. It is also worth noting that in all of 2007, only two advertisers have contacted us regarding click fraud data from Click Forensics, and in both cases we found that the suspicious activity was not charged for in the first place."

In addition to botnets, Cuthbert contends that parked domains are contributing to the alleged growth of click fraud. "Parked domains and made for ads sites are contributing as very large percentage of the low-quality networks in the ad networks," he said. Though he declined to provide more specific figure than "a very large percentage," he said he expected Click Forensics would eventually provide one.

Domain parking is the practice of putting ads on a placeholder Web page to collect pay-per-click fees generated by type-in traffic -- many people type search terms into the URL bar, forcing the browser to resolve them as domain names -- or search-driven traffic. Google offers AdSense for Domains to those who aim to monetize content-free sites, and it defends the service as useful.