For example, if the user frequently visits food sites, he or she might see an ad for a new cookbook.

Online networking sites have gone a step further by allowing for transmission of unique identifiers. It is a particularly troubling practice for two reasons, Wills said.

'First, users put a lot of information about themselves on social networking sites. Second, a lot of that information can be seen by other users, by default. There are mechanisms users can use to limit access to their information, but we found through previous research that most users don't take advantage of them,' added Wills.

With a unique identifier, a tracking site could gain access to a user's name, physical address, e-mail address, gender, birth date, educational and employment information, and much more.

With the 'leakage' of this type of personal information, there is a significant risk of having one's identity linked to an inaccurate or misleading browsing profile.

These results were presented in Barcelona at the Workshop on Online Social Networks, part of the annual conference of the Association for Computing Machinery's Special Interest Group on Data Communications.