The money has not been stolen from the fund. The fund was used to pay out current benefits and the surplus invested in bonds. The bonds are in the fund. They get redeemed for cash when they are needed to pay benefits.
http://dissidentvoice.org/2009/11/abuse-of-the-social-security-trust-fund-began-in-the-1980s/
And if we would have put it under a rock in the back yard, it would still be there wouldn't it? Better than letting the gov "borrow it" with a worthless IOU...they Stole It!!!