[lambda]

No, Social Security Numbers are not unique.

Furthermore, many sources of credit information do not have access to your social security number to use as a key. Your phone company, cable company, oil company, and so on all may need to send your bills to collections and put a mark on your credit report, but they don't have access to your social security number (they may ask for it, but you aren't required to provide it).

[lkbm]

There are cases of them being re-issued accidentally, but they're supposed to be unique, and we have massive data systems based on that premise. Texas has only just recently begun transitioning from using SSN as the primary key for their statewide student (and school staff) database.