Facebook population: 50 million+
The idea was simple enough: Facebookers who bought movie tickets on Fandango or booked a flight on Travelocity could broadcast that activity to their friends. By partnering with third-party Web sites, Facebook Beacon would usher in a new era of social advertising. The problem? Users balked at having their formerly private activities automatically made public, and security researchers found that Facebook still secretly collected this information even after users had opted out. Even today, Beacon is still synonymous with "privacy nightmare."
The Good: Facebook got solidly spanked for using members' data without asking
The Bad: Beacon later morphed into Facebook Connect and Timeline Apps