The Social Security trust fund is not like a typical private-sector trust fund. A typical trust fund can make a variety of investments; for example, stocks, bonds, real assets, mortgage-backed securities, commodities and precious metals. On the other hand, the Social Security trust fund can only invest its yearly surplus in special Treasury bonds. These bonds can only be issued to and redeemed by the Social Security Administration, which means they are not liquid assets i.e. the SSA or the trust fund cannot sell them in the open market in order to fulfill its obligations to retirees.
Critics argue that this nature of the social security trust fund means its assets aren't "real" like a bank account.
...extending the projected life of the trust funds neither requires nor guarantees that the surpluses are actually saved. The President's budget recognizes that the trust fund balances do not consist of assets in a bank account, to be drawn down in future times of need: These 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. The existence of large trust fund balances, therefore, does not, by itself, have any impact on the Government's ability to pay benefits.