Washington, Aug 25 - Personal information, shared by half a billion people on social networking sites, might be leaked to companies wanting to take advantage of their browsing habits and even specific identities, says a new study.

'When you sign up with a social networking site, you are assigned a unique identifier,' said Craig Wills, professor of computer science at Worcester Polytechnic Institute (WPI), who conducted the study with an industry colleague.

'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. Now your browsing profile is not just of somebody, it is of you,' added Wills.

Like most commercial web sites, online social networks use third-party tracking sites, called aggregators, to learn about the browsing habits of their visitors.

Cookies are maintained by a web browser and contain information that enable tracking sites to build profiles of the web sites visited by a user.

Each time the user visits a new web site, the tracking site can review those cookies and serve up ads that might appeal to the user.