Prior to June 25, 2011 the Social Security Administration (SSA) issued social security numbers in a method that was predictable and allowed for the numbers to be verified as having been issued or not. For example, the first three digits were a reliable indicator of what state the SSN had been issued in. There was also a monthly list of the highest numbers issued for each group of SSNs that had been issued.
As of June 25, 2011 the SSA switched to a new system called Randomization. The new system makes it impossible for services like this website to determine whether or not a SSN has been issued IF it was issued AFTER this date.
For further details on Randomization, please visit this page at the SSA website.
If you are an employer looking to verify the SSNs of employees, please visit the SSA page for Social Security Number Verification Service.
Please note this can only be used for very limited purposes to verify the SSNs of current employees.
If your business is looking for official verification of social security numbers of non-employees, the SSA has a program called Consent Based SSN Verification. This program has a large sign up fee plus a cost per verification but may be of interest to some parties. This option requires the signature of the party being verified on a designated form. More can be read about it here.