There aren't 9.3m fraudulent SSNs, there have been 9.3m invalid W-2s (ie SSNs don't match up) filed for 2005 tax returns. To my knowledge, there aren't any published stats available from the IRS - this was leaked info. As to the actual number of fraudulent SSNs floating around, it's probably incalculable.
Please parse your sentence for me as I don't seem to grasp the difference between an SSN that doesn't match (I'm guessing with a name in another DB) and one that is bogus.
It remains a starting point. If one employer has several "invalid SSN's" something smells, doesn't it?