Some people use a made up number, some others actually buy the SS card of somebody else to whom was legally issued to, and some others secure a legal SSN fraudulently via a fake birth certificate.
I find the latter part hard to believe! Presenting a fake birth certificate to the SSA people to obtain a legit SSN?! Do you know what penalties are in place for doing that?! Would you have the balls to actually go some place and ask for that?!
We've sure have heard about people from foreign countries (Mexico, being the obvious example) having an easy time submitting altered birth certificates (not totally fake, they just might have needed to change, e.g., their birth year, so that they'd fall within a certain age-limit in order to qualify for a particular benefit). In these cultures that's something quite 'doable', so to speak, with legal repercussions in case of detection being miniscule.
But not in the States!