A ponzi scheme is quite different. You pretend to invest an initial investor's money. The returns to that investor are paid out of money coming in from later investors.
Social security works in this way. The money paid into social security is not invested at all. The returns from social security (money going out to current retirees) come entirely from new investments (money going in from current workers). This is the basic definition of a ponzi scheme. It is a sustainable ponzi scheme so long as taxes collected from current workers exceed benefits paid out to current retirees. The designers of this system did not count on two things: (a) ever increasing life expectancy and (b) the baby boom. The former has increased the length of time each person collects benefits and the latter has resulted in a demographic bulge that is now hitting its retirement years.
Thank you Mr. Bismarck![0]
[0] https://en.wikipedia.org/wiki/Otto_von_Bismarck#Old_Age_and_...
I understand the comparison to Ponzi schemes, but I consider these comparisons rather hyperbolic. The intent of a pay-as-you-go pension never is fraud. There are some definite problems with pay-as-you-go systems, especially now due to the fertility decline in the West, but the issues do not stem from pure malice.
A ponzi scheme has an intent to defraud. That's why they're illegal.
Social security has no intent to defraud. It's not "legal cause the gubmint says so," it's legal because it's not a fraudulent money-making scheme designed to enrich the leader.
Social Security and other pay-as-you-go schemes aim to reduce poverty, especially elderly poverty. By this measure, it has succeeded quite well (http://www.nber.org/bah/summer04/w10466.html), and the probable reason pay as you go was chosen I imagine was to provide immediate benefit (recall this scheme was launched during the Great Depression). I would consider it radical (and would want one to provide considerable justification) if one wanted to ditch Social Security entirely.
A better debate in my mind, of course, would be whether to phase Social Security into a fully funded model versus pay-as-you-go model. I can buy that kind of argument, a fully funded retirement scheme is more secure than a PAYGO type.
But even as it stands, the current Social Security gap doesn't sound like a complete disaster. Part of it actually is because Social Security is not completely pay as you go -- Congress raised payroll taxes in 1983 in part to cover this gap (http://www.pewresearch.org/fact-tank/2015/08/18/5-facts-abou...). Part of it is because we have some variables to play with -- raising the retirement age or raising the payroll tax cap, for instance. Medicare actually sounds like its in worse shape at this moment.
Seriously, just google "is social security a pyramid scheme".
You'll find a LOT of breathless articles from personal blogs and sites like World Net Daily calling it a pyramid scheme.
You'll also find a lot of, for lack of a better way to put it, content of actual substance that describes how it's not a pyramid scheme, but a sustainable government program that's worked out just fine for decades (with regular adjustments, as all human endeavors need).