How unique are SSNs?
According to MSNBC, a study by San Diego start-up ID Analytics indicates that there's a significant chance that your Social Security number is being used by someone else. This report apparently claims that this happens to about one in seven people.
I couldn't find a copy of the report that MSNBC referred to to see exactly what ID Analytics was measuring to get this estimate. It's certainly not easy to find on their web site. So at this point I'm inclined to interpret this report as an attempt by a company that sells an ID-monitoring service to make fraudulent use of identities sound more common than it really is to get media attention and to encourage sales of their services.
The same MSNBC article quotes someone from credit reporting firm Experian who says that most of these cases are caused by an honest mistake – someone mistyping their SSN, for example, instead of intentionally using the wrong SSN, and that Experian's systems catch most of these errors. So your SSN may indeed be used by someone else right now, but it's probably just because of an unintentional error instead of something that should be called "identity theft."