So how common is online behavioral tracking? In a path-breaking series of articles in the Wall Street Journal in 2011, researchers examined the tracking files on 50 of the most popular U.S. Web site. What they Found revealed a very widespread surveillance system. On the 50 site, they discovered 3,180 tracking files installed on visitor computers. Only one site, Wikipedia, had no tracking files. Some popular sites such as Dictionary.com, MSN, and Comcast, installed more than 100 tracking files! Two-thirds of the tracking files came from 131 companies whose primary business is identifying and tracking Internet users to create consumer profiles that can be sold to advertising firms looking for specific types of customers. The biggest trackers were Google, Microsoft, and Quantcast, all of whom are in the business of selling ads to advertising firms and marketers. Google, given its dominance in search, knows more