Is there or is there not an actual Social Security trust fund?
Since the mid-1980s, the Social Security Administration (SSA) has been collecting more in payroll taxes each year than it pays out in pension, survivor, and disability benefits. The difference between receipts and payments grew significantly in the 1990s, and now amounts to some $160 billion each year. The Social Security system is expected to continue running annual surpluses at least through 2025.
Each year, SSA turns over any surplus funds to the U.S. Treasury, which spends the funds. In return, SSA receives special-issue, non-negotiable U.S. Treasury securities, which represent an implicit promise by the U.S. government to repay Social Security when and if additional money is needed to cover benefits. These bonds are what we call the "trust fund." In 2000, the trust fund contained bonds valued at $1.2 trillion; by 2025, the accumulated surpluses should top $3 trillion.
These, of course, are projections—the surpluses (and thus the trust fund) could be larger or smaller than anticipated, depending on wage growth, population changes, the overall state of the economy, and so on. Under the SSA's "low-cost" (or best-case) scenario, the Social Security trust fund will grow continuously until late in the 21st century.
So, yes, there is a trust fund, representing the excess of payroll taxes over benefit claims, and it is "invested" in promissory notes issued by the government