Any overage beyond current needs is put into a "trust fund" which is required by law to be kept in US Treasury Bonds, aka loaned to the US Government. For the truly cynical, think about it as years of loaning a bunch of money to your uncle, and around the time that money starts needing to be paid back your uncle starts looking for contract assassins (aka "privatization"). If Social Security can be killed then oh my! Guess all that money owed to it just doesn't need to be paid back.
If the money "borrowed" in that way had been spent in ways that would make providing the services it's for easier and more cost effective that would be one thing, but that's not how it works out because the best ROI for private capital is purchasing politicians and policy.
Sure, in one case the parties are me and the US Department of the Treasury and in the other case it’s the SSA and the US Department of the Treasury, but I don’t really see how this matters.
>Sure, in one case the parties are me and the US Department of the Treasury.
It would be the same situation if the DOT taxed you when the T-bills come due.
Trade 1$ for a treasury today. Tomorrow get taxed $1.1 and paid back with your own money.
It’s the same concept as a trust, but essentially at scale for multiple parties.