The H has an article about some researchers who found a new way to de-anonymize people. Compared to the EFF's Panopticlick, the goal of this experiment is not to identify a user's browser uniquely, but to identify individual users.
The test essentially exploits the fact that many social network users are identifiable by their membership of various groups. According to the researchers, it's very unlikelly that two people on any social network will belong to exactly the same groups. A 'group fingerprint' can thus allow websites to identify previously anonymous visitors. They describe the setup and all details and the results look very interesting.
They also have a live demo for the social network Xing that was able to de-anonymize me.