The essential element of a ponzi scheme is that investors are deceived.

> A Ponzi scheme is a form of fraud that lures investors and pays profits to earlier investors with funds from more recent investors. The scheme leads victims to believe that profits are coming from legitimate business activity (e.g., product sales or successful investments), and they remain unaware that other investors are the source of funds.

In contrast, the Social Security model is pretty transparently a wealth transfer from workers to the elderly and disabled.