Q: How is Social Security different than a Ponzi scheme?
A: Ponzi schemes are voluntary.

A Ponzi scheme generally falls apart when there aren’t enough new investors suckers for inflows to cover outflows. But when you’re the government, that’s not really a problem: you just change the rules of the game, raising the contributions of the investors suckers taxpayers, raiding general revenue, etc. They can do this, of course, because as long as they have a modicum of public opinion on their side, this can easily be accomplished by coercion, or more plainly the threat that men with guns will come to your house and commit lawful violence against you if you don’t go along.

So, yes, the comparisons of Social Security to a Ponzi scheme are grossly unfair — to Ponzi schemes.