"Social Security taxes are paid into the Social Security Trust Fund maintained by the U.S. Treasury. Current year expenses are paid from current Social Security tax revenues. When revenues exceed expenditures, as they have in most years, the excess is invested in special series, non-marketable U.S. Government bonds, thus the Social Security Trust Fund indirectly finances the federal government's general purpose deficit spending."

http://en.wikipedia.org/wiki/Social_Security_(United_States)...