And not all 9-digit numbers are used, so there are fewer than a billion. It sucks when organizations store them because the search space is so small it's relatively easy to unhash them in a stolen database.

9 digit social security number specifically might be, but a unique number tied to you that is often used as identification when it really shouldn't isn't, it's a shitshow that has been implemented in many countries around the world.
The Finnish version was called an SSN originally for example, though now its a "henkilötunnus", personal identity code.

https://en.wikipedia.org/wiki/National_identification_number

Do TINs overlap with SSNs? Because businesses and non-citizen taxpayers have TINs instead of SSNs, but they're used just the same.