The weird difference between US SS# and Canadian SIN# is who, by law, by can demand that you give them the #.
In Canada, you don't have to give your number for credit check. It may slow down the process, and some companies may choose not to do business with you. You do have to give your SIN# to bank, so that they can tell the tax people about your taxable interest. But you don't have to give your SIN# to the federal govt (the people who gave you the SIN# in the first place) to receive services, unless they are giving you money. My wife used to work for the department that managed the system, and provided unemployment services (job clubs, etc) and every once in awhile someone would refuse to divulge their SIN#. Unless there was money involved (and therefore the tax department) they had to respect their privacy rights. Weird.