Social Security is more of an obligatory investment account where you put in principal and then you receive back your principal at old age. This is not, by definition, a Ponzi scheme because it's not paying you yields on your principal which you may withdraw.

The fact that you're putting it into an account which is used to pay someone else's retirement and then hoping someone else will pay yours is not precisely a Ponzi scheme, it is more like an insolvent bank.