A large portion of the federal budget consists of transfer payments from today's working population to retirees, with the expectation that tomorrow's working population will make similar transfer payments to today's.
How is transferring money from workers to non-workers is reminiscent of a Ponzi sceme in any way? There's no imaginary wealth being created, it's just a transfer of wealth from younger to older generation. I wouldn't bet that future young will be as silent about it as we are, but I wouldn't completely discard it as a "Ponzi scheme" either.
There is no guarantee that there will be enough workers in the next generation to cover the workers who are paying today. Without new recruits, the system fails to meet it's promises.