Many popular social networking sites typically make personal information available to companies that track users browsing habits and allow them to link anonymous browsing habits to specific people, according to a new study by the Worcester Polytechnic Inst

When you sign up with a social networking site, you are assigned a unique identifier, says Craig Wills, professor of computer science at WPI.This is a string of numbers or characters that points to your profile. We found that when social networking sites pass information to tracking sites about your activities, they often include this unique identifier. So now a tracking site not only has a profile of your Web browsing activities, it can link that profile to the personal information you post on the social networking site.