Unlike private pension funds, the Social Security Trust Fund does not hold any marketable assets to secure workers' paid-in contributions. The Social Security Trust Fund "invests" surpluses in United States Treasury bonds and U.S. securities backed "by the full faith and credit of the government".
Practically speaking, the federal government has used the Social Security Trust Fund in a way which would be illegal for any private-sector company to do – in order to help balance the federal budget, the government has borrowed money from the Trust Fund to pay current operating expenses and replaced those funds with government IOU's.
The relationship between the Social Security Trust Fund and the national debt requires some explanation. As of the early 21st century, the amount raised by the Social Security taxes are in excess of the payments to recipients of social security. Hence in calculations of government spending, it is tempting to include those budget surpluses in government accounts. However, this is problematic because Social Security will go into deficit by 2015, and hence if Social Security surpluses are used to balance the current budget, extra funds will be needed when Social Security goes into deficit.
The FY 2000 budget put the issue very clearly:
These [Trust Fund] balances are available to finance future benefit payments and other Trust Fund expenditures – but only in a bookkeeping sense. ... 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. The existence of large Trust Fund balances, therefore, does not, by itself, have any impact on the Government’s ability to pay benefits. Source: FY 2000 Budget, Analytical Perspectives, p. 337