My recently found out that my ex-employer withheld FICA from all my checks along with everybody else's but never sent the money to SS, what are we supposed to do?

Call the IRS fraud hotline so they can come and arrest your employer. OK, I'm being a little dramatic here - they probably won't actually arrest him but they will make him pay the back taxes plus interest and a fine. See: How Do You Report Suspected Tax Fraud Activity?