Once they kept track only of who was eligible for old-age pensions. Today they're used to validate personal information for a multitude of transactions - from renting a video to opening a credit account to receiving medical care.
Seventy years old this month, Social Security cards, each with its unique identifying digits, have become an integral, nearly indispensible, part of American life and commerce.
But their increased use has also made the numbers valuable for identity thieves, who can use the information to empty someone's bank accounts or run up credit-card charges. As awareness of the problem has grown, Social Security numbers have begun to disappear from driver's licenses and other highly visible documents, and businesses are guarding them more carefully. Many people no longer carry their Social Security card in their wallet or purse.
While these efforts are laudable, privacy experts say, they're too little, too late. "Reducing the collection and display of Social Security numbers is a small help," says Robert Gellman, a privacy and information policy consultant in Washington, D.C.
"The vast majority of the American populace [already] has their Social Security number out where any identity thief can obtain it with remarkable ease," adds Timothy Sparapani, who tracks privacy rights issues for the American Civil Liberties Union (ACLU).
Unlike a computer password, a Social Security number is difficult to change. The process usually involves persuading the Social Security Administration that you have been a victim of identity theft - and that you're not trying to hide from creditors or a criminal record.
There's much talk of legislation to limit the use of Social Security numbers by private industry, Mr. Gellman says, but inevitably various groups will ask for exceptions, and any new law is likely to be watered down. What legislation might profitably do, he says, is try to protect future Social Security numbers from being compromised. For numbers that already have been issued, it's "largely a lost cause," he says.