One of the biggest users of social security numbers for identification is the US military. When my dad was in the army, my mom and I both had his SSN memorized because we had to use it so often on paperwork.
I've always worried that enemy forces would capture our troops, steal their identities, and then ruin their credit.
Anyway, I have access to the SSN of just about everyone in the army over the last five years. The do give the special forces people substitute SSN numbers but other than that it's fairly easy information to get access to if you if you are doing any sort of analysis of army trends.