Few people would classify legalized theft as an investment. An investment is generally considered to be voluntary.
If A is an investment broker.
However. . . .
B invests $100 with A expecting X return on the money.
But rather than growing the $100, A instead spends the money and takes money from C in order to pay B.
Can you say Bernie Madoff?
Now which of these two definitions looks more like Social Security?
Some people voluntarily write checks to the SSA, granted they would go to jail if they refuse.
While I agree investments are generally voluntary, there's no denying that there is a rate of return on SS deposits for some lucky few. Money goes into an account, money comes out. Some people loose more than they put in some get more than they put in. Most investments have a variable rate of return, many if not most can actually loose money. It's the same damn thing with SS. The only real difference is, in this case government mandates it by law, unless you get an exemption. Exemptions include priests, I believe one religion, and some federal union employees.
By design, it started out as a ponzi, and still is a ponzi. Each and every successive generation has had to pay twice as much as the previous generation by % of income. This pyramid proves without a shadow of doubt that it is a ponzi. The only final nail in the coffin is the last generation getting left holding the 50t + bag and nothing but IOUs in return.
It is well known that the only way to "save" this program is to cut the benefits, again, and/or increase the amount taken.