The key bit is that it (sounds like) they use it only for identification, whereas the SSN in the US is treated as a "secret" which can be used for authentication (!?). Thus a weird legal limbo where it's officially not supposed to be shared around, as it can prove who you are... yet all kinds of businesses are required to record it to identify their employees :|
Not quite the same thing, but ID Analytics claimed that "6.1 percent of Americans have at least two SSNs associated with their name," and "more than 15 percent of SSNs are associated with two or more people" in commercial records.
Right, that goes along with "20% of credit reports have errors." (which was found in an FTC study). Example errors are misspellings and number transposing and both versions end up in the database. The correct report can be pulled up 99.9999% of the time even if it has errors because usually a bunch of data is used to pull up the report (name, address, birthday, social), not just a single number.
I know someone with misspellings of their name on one bureau's credit report and its never caused a problem (he's too lazy to ask them to fix it). The misspelling is listed as his name and the right spelling is listed as "other name" (or something like that).
The keyspace is also shared with the larger "TIN" (Tax ID Number) pool, though the pretty-print format is different:
SSN and ITIN: nnn-nn-nnnn
EIN: nn-nnnnnnn
So it covers not only all the individual taxpayers but also all the employers and businesses that are taxable separately from individual income tax payers.
So ... how can it not be exhausted yet, or at least close enough that it's irresponsible for officials to offer blanket denials regarding reuse of defuncts?