A "Ponzi" is a investment scheme (the word has negative connotations but none are intended here) that utilizes current receipts to pay for current benefits. This is what SS does. All the misinformed folks who say "Don't mess with MY Social Security, I paid for it, it isn't an entitlement." are simply flat out wrong.

I am paying for my father's Social Security check, just like he paid for his father's, and my kids will be paying for mine.

It is a Ponzi, because of how it is structured, not if it has worked in the past, or can continue to work in the future or how large the pool of forced contributions is. How big the forced contribution is, versus how pitiful the payout is the ONLY reason it has lasted this long.