I’ve written about this repeatedly in the past, but here it is again: Social Security is a program that is part of the federal budget, but is by law supported by a dedicated source of revenue. This means that there are two ways to look at the program’s finances: in legal terms, or as part of the broader budget picture.
In legal terms, the program is funded not just by today’s payroll taxes, but by accumulated past surpluses — the trust fund. If there’s a year when payroll receipts fall short of benefits, but there are still trillions of dollars in the trust fund, what happens is, precisely, nothing — the program has the funds it needs to operate, without need for any Congressional action.
Alternatively, you can think about Social Security as just part of the federal budget. But in that case, it’s just part of the federal budget; it doesn’t have either surpluses or deficits, no more than the defense budget.
Both views are valid, depending on what questions you’re trying to answer.