Bernie Madoff took money from people who thought he'd invested it, gave some to others who thought it was a partial return on their earlier investments, and kept much for himself. That's called a Ponzi scheme, and his $50 billion fraud was called the biggest ever. But it wasn't the biggest. Social Security and Medicare are much bigger ones.
These are trillion-dollar scams. Medicare has a $36 trillion unfunded liability. Social Security's is $8 trillion. There's no money to keep those promises.
But Congress isn't investigating this scam. Congress runs it. That FICA money you thought government had saved for your retirement is gone. There's nothing left but IOUs backed by nothing. Your money was spent not only on current retirees but on wars, welfare, corporate bailouts, earmarks, and all the other stuff Congress wants. For years, this was possible because the FICA tax brought in surpluses that allowed government to pay retirees more than they contributed and still help buy those other things.
Read the whole thing, if you can bear it.