When the Federal government started issuing Social Security cards five decades ago, some people worried the Social Security number would evolve into an all-purpose, national identification system. They were right. These days, the number tracks everything from college students to video-store customers, and it's virtually impossible to get by without one. Still, many Americans resist it, believing the number has the power to unlock personal information. Privacy experts say people are right to guard their Social Security numbers, but they also warn that keeping the number secret is no guarantee of privacy.