I don't think most folks understand this. It's true and it's the real reason the rats are so upset about private accounts--private accounts reduce the flow of SS taxes going to the government and put them in private accounts instead.
Currently, when SS taxes come in, the government immediately borrows those taxes from the trust fund and spends them, leaving an IOU. When the boomers retire and congress goes to the pantry to pay their SS, a big pile of IOU's from congress will fall out. You can thank LBJ and the great society for that abomination.
And that's also why increasing the SS taxes will do nothing to improve the solvency of Social Security. The increased revenues will just be borrowed and spent. The result, a bigger pile of IOU's in the pantry.