I found this explanation of the Social Security Trust Fund and its differences from a legitimate trust fund, by Heritage's David John, to be useful.
How the Trust Fund Operates.Workers pay their Social Security taxes through their employers. Each employer periodically sends a lump sum payment to the U.S. Treasury that includes all of the income taxes and Social Security and Medicare payroll taxes paid by both the employer and its employees.
The Treasury both receives the payroll taxes (and income taxes that higher-income retirees pay on their Social Security benefits) and pays monthly benefits on behalf of the Social Security Administration (SSA). The money stays in the Treasury's hands until it is either paid out as Social Security benefits or otherwise spent by the government. In fact, no money ever goes into the trust fund. Instead, the trust fund balance is the result of two accounting entries by the Treasury.
First, the Treasury estimates how much of the aggregate tax receipts are Social Security taxes and “credits” the Social Security trust fund with that amount. Then the Treasury “subtracts” the total amount paid in monthly Social Security benefits from the trust fund balance. No money actually changes hands; these are strictly accounting entries.
Any “money” remaining in the trust fund is converted into special-issue Treasury bonds, which are really nothing more than IOUs. In addition, the Treasury pays interest on the trust fund's balance by crediting the trust fund with additional IOUs. These are also strictly accounting entries, and again no money changes hands. After crediting the trust fund with the proper amount in IOUs, the government spends the extra Social Security tax collections just like any other tax revenue–to finance anything from aircraft carriers to education research.
At the end of 2002, the Social Security trust fund had a balance of $1.22 trillion. During 2003, the Treasury received $544 billion in Social Security taxes and paid out $406 billion in Social Security benefits. Therefore, the trust fund received $138 billion in these special-issue Treasury bonds, resulting in a trust fund balance of $1.36 trillion at the end of 2003.
Why the Social Security Trust Fund Differs from Real Trust Funds. Private-sector trust funds invest in real assets ranging from stocks and bonds to mortgages and other financial instruments. However, the Social Security trust funds are only “invested” in a special type of Treasury bond that can only be issued to and redeemed by the Social Security Administration. As the Congressional Research Service noted in a report on May 5, 1998:
When the government issues a bond to one of its own accounts, it hasn't purchased anything or established a claim against another entity or person. It is simply creating a form of IOU from one of its accounts to another.
According to the Office of Management and Budget under the Clinton Administration in 1999:
These [trust fund] balances are available to finance future benefit payments and other trust fund expenditures–but only in a bookkeeping sense. These funds are not set up to be pension funds, like the funds of private pension plans. They do not consist of real economic assets that can be drawn down in the future to fund benefits. Instead, they are claims on the Treasury, that, when redeemed, will have to be financed by raising taxes, borrowing from the public, or reducing benefits or other expenditures. [Emphasis added.]
In short, the Social Security trust fund is really only an accounting mechanism. The trust fund shows how much the government has borrowed from Social Security, but it does not provide any way to finance future benefits. The money to repay the IOUs will have to come from taxes that are being used today to pay for other government programs. For that reason, the most important date for Social Security is 2018, when taxpayers must begin to repay the IOUs, not 2042, when the trust fund is exhausted.
The point is, if the government abolished the trust fund and threw all the special issue securities in a big pit and burned them tomorrow, the financial picture of Social Security would be exactly the same.
[Should we call those who tout the massive assets of the trust fund “trust fundamentalists”?]