The surplus collected into social security, which was supposed to be set aside and invested for the time when there'd be greater demands on it, was instead spent, replaced with an empty IOU.
Another point that's worth bearing is that these IOUs aren't just empty: they're meaningless. To understand this, we have to understand that people are not entitled to social security payments; it is not like a contract where you pay into the system, and then have a legal right to get something back out later on.
Nor is social security some form of insurance.
If we were to talk of real privatization, it would mean eliminating the program entirely, and allowing a combination of voluntary savings, voluntary insurance, and voluntary charity to work in the free market. However, what is actually being talked about is nothing other than the creation of a forced-savings program.
Also worth a look is the No More Euphemisms article mentioned in that post.
Social Security as it is currently structured has nothing to do with legally enforceable promises or guarantees. There is no "trust fund" as that term is commonly understood, no funded segregated accounts, no IOUs or bonds stored in some lockbox, or anywhere else for that matter. Social Security is neither solvent nor bankrupt.