Sure, you keep your Social Security number confidential. However, according to researchers from Carnegie Mellon University, it's possible for crooks to guess most of the digits in the Social Security number of many Americans using publicly-available information.
Researchers Alessandro Acquisti and Ralph Grossy took advantage of a couple of practices of the Social Security Administration. Since its inception, there has been a pattern to the issuance of numbers. The first three indicate the state in which you lived when you received your number. The two in the middle, called group numbers, are assigned in a known sequence. The final four are also assigned sequentially, 0001 to 9999.