In exchange for the payroll taxes that aren't paid out in benefits to current retirees in any given year, the trust funds got nonmarketable Treasury debt.
WSJ: Cox and Archer: Why $16 Trillion Only Hints at the True U.S. Debt
2.
Now, as the baby boomers' promised benefits swamp the payroll-tax collections from today's workers, the government has to swap the trust funds' nonmarketable securities for marketable Treasury debt.
WSJ: Cox and Archer: Why $16 Trillion Only Hints at the True U.S. Debt