In its early days, the SSN wasn't widely treated as sacrosanct. In 1938, a wallet manufacturer in New York, which wanted to advertise how well those new Social Security cards fit into its billfold, used the actual number of its treasurer's secretary, one Mrs. Hilda Schrader Whitcher. Mrs. Whitcher's secret identifier (078-05-1120) was soon on display at Woolworth and other department stores around the country.

By 1943, nearly 6,000 people were using her number, according to the Social Security Administration, which voided it. Over the years, more than 40,000 people claimed the number as their own, and 12 people were found to be using it as late as 1977. 

"They started using the number. They thought it was their own," the real 078-05-1120 said, according to a history on the Social Security Administration web site. "I can't understand how people can be so stupid."

Since then, many companies and government agencies, while using SSNs internally, have at least stopped displaying them on ID cards and using them as subscriber numbers. Many use unique numbers, sent to a recognized device such as a cellphone, in place of the familiar request for the last four digits of the Social.  Some have suggested creating individual encryption keys, sort of like the code-generating tokens that workers use to access their computers from outside the office. Another idea, a national identification card, "creeps people out, because it seems very Orwellian," Velasquez said.

Even creepier, she said, one frustrated consumer proposed that the government "just microchip me so you can scan me and thieves can't dig it out of me." 

Until we all get chipped, the only person who can sharply curb the use of your Social Security number is you.

"Don't blindly provide it because you're asked for it," said Gary Davis, chief consumer security evangelist for anti-virus software provider McAfee. 

The tricky part is that you can be denied service. The Social Security Administration recommends asking why the number is needed, how it will be used, what will happen if you refuse to give it, and what law requires you to give the number to a private business. For example, there's no legal reason you must give it to your doctor. Doctors almost always ask for it, though, sometimes because they're using outdated forms, or for patients on Medicare, since your Medicare number is your Social Security number.

On income tax forms and financial accounts that wend their way to the Treasury Department, the ritual asking for and giving of the Social Security number is all but inevitable. Same with food stamps, child support enforcement programs, and state commercial driver licensing programs. Credit bureau TransUnion says the nine-digit wonder is indispensable. 

"We consider the SSN to be an important part of the consumer reporting and credit granting ecosystem, and many regulators and consumer advocates recommend this approach, where available, for accurate matching," TransUnion spokesman Dave Blumberg said in an email. "The SSN is also an important tool in identity verification and can help lenders to detect and prevent identity theft."