This is changing because of banner ads served by central sites, such as Adknowledge, MatchLogic, and DoubleClick, the largest Internet ad agency. Doubleclick places ads on about 1,800 Web sites, including some of the most popular. A cookie set by a DoubleClick ad server can be read by any server in its domain, no matter what site you are at or what is on your screen. Thus the cookie can probably describe the page where the ad was seen, and track your activities across dozens of sites served by the agency. As a result, you see still more banner ads targeted to your taste.
Marketers argue that because anonymous machines are involved, no harm is done.
This, too, is changing. Last year, DoubleClick bought a huge database with records of 88 million households that buy from catalogs. The company now plans to match the supposedly anonymous information from cookies with your name, address, telephone number, age, sex, income level and history of purchases. Although DoubleClick is now under investigation by federal and state agencies, the company denies any wrongdoing. Privacy advocates fear that not only browsing habits or research interests but also health information could be released to employers, insurers, and others.