When users clicked ads on Facebook, Web links sent to advertisers contained unique IDs that could be traced back to the users' public profiles--giving the advertisers access to detailed information about a user's . In other cases, to brokers.

EFF's Peter Eckersley says using Facebook IDs to extract personally identifiable information is easy for data brokers. "Tracking people is what they do," he says. "If they're sitting on a gold mine of data, they're going to dig for gold."

The solution: Use Facebook's privacy controls to keep your public profile sparse, and when possible.

Threat Level: ORANGE