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.
Most social security payment are saved as government bonds. 
mnli hypothesis: Most social security payment are saved as government bonds.  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.