I have one - it is shared by at least four others in various parts of the country. Guess who told me that - the Social Security people. I asked what they were going to do about it. The answer was “Nothing.”
Probably just following orders from someone higher up who doesn't want such crimes investigated, let alone prosecuted. That lack of enforcement goes throughout our bureaucracies and it will take a long time to turn it around.
And we know who's being protected by the lack of enforcement of that, and so many immigration laws.
How did you learn that you SS# was being used by four others?