It IS a scheme, in that it's a system, plan, or action.
It is NOT a Ponzi Scheme because it does not promise investment returns or exaggerate profitability.
It's there to keep retirees & the disabled out of abject poverty, which does not provide returns you can usually see on a balance sheet. Greater good & all that...
Social Security is an investment in the health of our society, it's not a financial instrument. You can't pool risk and get predictable returns without cutting corners & dropping high-risk members (hello health insurance industry).
It's not a Ponzi Scheme by definition.