Norms are changing, with confidentiality giving way to openness. Participating in YouTube, Loopt, FriendFeed, Flickr, and other elements of modern digital society means giving up some privacy, yet millions of people are willing to make that trade-off every day. Of people with an online profile, nearly 40 percent have disabled privacy settings so anyone may view it, according to a Pew Internet survey released a year ago. The percentage is probably higher today.
In social networks, people can increase their defenses against identification by adopting tight privacy controls on information in personal profiles. Yet an individual’s actions, researchers say, are rarely enough to protect privacy in the interconnected world of the Internet.
You may not disclose personal information, but your online friends and colleagues may do it for you, referring to your school or employer, gender, location and interests. Patterns of social communication, researchers say, are revealing.
“Personal privacy is no longer an individual thing,” said Harold Abelson, the computer science professor at M.I.T. “In today’s online world, what your mother told you is true, only more so: people really can judge you by your friends.”
Collected together, the pool of information about each individual can form a distinctive “social signature,”