I truly think you made a big mistake.
OK so I applied for a job at a store at the mall but I used a fake social security number. Do you think they can notice if its fake? because I know a lot of people who don't have a ssn but still work so they most likely give a fake number...?
Yes they will know its fake they're not stupid!
Short term thinking. Even if you do get away with this now the far future will not greet you with a fall back retierment income from social security.
A fake social? That's very dangerous. If you do get away with it, it will be difficult to pull things off afterwards. Truly you should have told them you didn't have a social.