From The Patriot Post:
Strictly speaking, Social Security is not a Ponzi scheme, in part because it’s not against the law. Indeed, it is the law. (Try not paying payroll taxes — i.e., “investing” in the system.) But it is structured exactly like a Ponzi scheme, and it will eventually fail for the same reasons. Today’s workers are paying for the checks of today’s retirees, and it has always been that way. From the start, politicians have raided the Social Security “trust fund” and spent the money on other general fund projects. What was left were worthless IOUs. Now that benefits paid exceed taxes collected, that problem has become acute. According to the Social Security board of trustees, in 1945, there were 42 workers for every retiree; the current ratio of three workers to every retiree is unsustainable.
Emphasis added.