told their payout relies entirely on contributions from future investors... the premise of the ponzi scheme is that there is a legitimate underlying investment. But there is no underlying investment so the only way investors can can paid is if more and more investors are attracted. Eventually, the ability to continue to attract an increasing number of investors wears out and the ponzi scheme collapses. Ponzi schemes also collapse when investors try to withdraw their investment all at once (say in a market correction)

Social security is very up front about the structure of the program. Current workers know they are paying into a system and earning a future benefit to be paid by future workers. So there's no fraud involved in social security which is a fundamental component of a ponzi scheme. There's also no risk of a "run" on social security where everyone tries to withdraw their investment at once. There IS no withdrawal of one's investment. And finally, without ANY changes to the current tax and benefits structure of social security, social security can fund 85% of all future benefits indefinitely. With a few tweaks to the tax and benefit structure, you could have a program that is indefinitely solvent. That's not at all the case with ANY Ponzi scheme.