Coruscation

Computer scientists and policy experts say that such seemingly innocuous bits of self-revelation can increasingly be collected and reassembled by computers to help create a picture of a person's identity, sometimes down to the Social Security number.

"Technology has rendered the conventional definition of personally identifiable information obsolete," said Maneesha Mithal, associate director of the Federal Trade Commission's privacy division. "You can find out who an individual is without it."

Carter Jernigan and Behram Mistree analyzed more than 4,000 Facebook profiles of students, including links to friends who said they were gay. The pair was able to predict, with 78 percent accuracy, whether a profile belonged to a gay male.

On Friday, Netflix said that it was shelving plans for a second contest -- bowing to privacy concerns raised by the F.T.C. and a private litigant. In 2008, a pair of researchers at the University of Texas showed that the customer data released for that first contest, despite being stripped of names and other direct identifying information, could often be "de-anonymized" by statistically analyzing an individual's distinctive pattern of movie ratings and recommendations.

pair of researchers that cracked Netflix's anonymous database: Vitaly Shmatikov, an associate professor of computer science at the University of Texas, and Arvind Narayanan, now a researcher at Stanford University.

By examining correlations between various online accounts, the scientists showed that they could identify more than 30 percent of the users of both Twitter, the microblogging service, and Flickr, an online photo-sharing service, even though the accounts had been stripped of identifying information like account names and e-mail addresses.