Social Security is an unfunded pay-as-you-go system--most of the payments into the system are transferred to current retirees for current consumption, not saved as government bonds.
Current retirees can directly use most of the payments. 
mnli hypothesis: Current retirees can directly use most of the payments.  premise: Social Security is an unfunded pay-as-you-go system--most of the payments into the system are transferred to current retirees for current consumption, not saved as government bonds.