Those U S Treasury notes that were put in the till when the money was drawn out are merely IOUs. No money in them, only accounting entries. The money was spent.
Yes and no.
Yes: That's how the SS trust fund money was spent. By law, the only thing the SS money can be invested in are U.S. Treasuries. So Congress didn't technically raid the SS fund and spent it (answering FReeper reganator's original quesiton). What happened is Congress ran up debt knowing that someone would be buying a bunch of U.S. treasuries (the SS fund). Until recently, for many decades the SS fund was the largest owner of U.S. debt, until the Federal Reserve increased its balance sheet and bought a bunch of treasures (through QE or Quantitative Easing). But I think the SS fund is now back to being the largest U.S. debt holder (after the Fed lowered its balance sheet with QT Quantitative Tapering).
Here's why I answer partly in No.
No: It's a stretch to say that treasury notes are just accounting entries with no money. Mine and my wife's retirement investments (mostly in our Roth IRAs) are spread out across 40+ mutual funds of different asset classes, 1/4th of which are bond/treasury/money market type (to handle market downturns). The two treasury funds (one long-term, one intermediate) both grow more than they shrink and I make some money in them. Bonus points that they tend to grow a lot when the stock market crashes. Are treasuries worth making the core of one's investments (of course not). And they certainly don't grow enough to prop up the Ponzi scheme called Social Security, especially with the left's push half a century ago to lower birth rates to save us from over population .... after they set up a Ponzi scheme that depended on sky high fertility rates to bring new "investors" into SS.
Never forget that they had to change the law so they could drain it.