You can’t get your money back. Your money is gone. What you get is other people’s money.
When I put my money in the bank, they don't write my name on the specific bills I give them, put them in a vault and give me back the exact same bill I put in years ago.
When I pay taxes into SS (a bad deal for most workers by the way) what they give me is a claim against future SS taxes.
While the truth is that they squandered the SS Trust Fund long ago, I still have a valid claim against SS and their current income. Only the government can legally operate a ponzi scheme like this, but they can and do. I also realize that they can change my claim at the whim of Congress, but there are political consequences which resist this.
So, it is as much "my" money as a bank deposit. (As we have seen in Cyprus and other select countries, bank deposits can easily be taken by government.)