"Taxes paid by today’s workers are used to pay today’s retirees. If money is left over, it finances other government spending—though, to maintain the insurance fiction, paper entries are created in a “trust fund” that is simultaneously an asset and a liability of the government. When the benefits that are due exceed the proceeds from payroll taxes, as they will in the not very distant future, the difference will have to be financed by raising taxes, borrowing, creating money, or reducing other government spending."
Basically politicians (of both parties) raid the Social Security fund and just leave IOU notes behind to fund their pork projects. If the money was untouchable and only used for Social Security, the problems we'd see with just population changes wouldn't be nearly as bad... hence (I could be wrong but I feel) this is one of the core arguments that Social Security is a Ponzi scheme.