Most Americans believe that the Social Security trust fund contains a pot of money that is sitting somewhere earning interest to pay their benefits when they retire. On paper this is true; somewhere in a Treasury Department ledger there are $2.4 trillion worth of assets labeled "Social Security trust fund."
The problem is that by law 100% of these "assets" are invested in Treasury securities. Therefore, the trust fund does not have any actual resources with which to pay Social Security benefits. It's as if you wrote an IOU to yourself; no matter how large the IOU is it doesn't increase your net worth.
This fact is documented in the budget, which says on page 345: "The existence of large trust fund balances … does not, by itself, increase the government's ability to pay benefits. Put differently, these trust fund balances are assets of the program agencies and corresponding liabilities of the Treasury, netting to zero for the government as a whole."
Consequently, whether there is $2.4 trillion in the Social Security trust fund or $240 trillion has no bearing on the federal government's ability to pay benefits that have been promised. In a very technical sense, it would lose the ability to pay benefits in excess of current tax revenues once the trust fund is exhausted. But long before that date Congress would simply change the law to explicitly allow general revenues to be used to pay Social Security benefits, something it could easily do in a day.
The trust fund is better thought of as budget authority giving the federal government legal permission to use general revenues to pay Social Security benefits when current Social Security taxes are insufficient to pay current benefits--something that will happen in 2016. Effectively, general revenues will finance Social Security when the trust fund redeems its Treasury bonds for cash to pay benefits.
http://www.forbes.com/2009/05/14/taxes-social-security-opinions-columnists-medicare.html