"Some in our country think that Social Security is a trust fund – in other words, there's a pile of money being accumulated. That's just simply not true. The money – payroll taxes going into the Social Security are spent. They're spent on benefits and they're spent on government programs. There is no trust." G. Bush

The government has borrowed nearly $2.8 trillion as of 2014 from the Trust Fund and used the money for other purposes. Wikipedia

What the government has done it has taken the excess money ($2.83 trillion) from SS taxes, and spent the money and replaced it with US Treasury bonds. It's a slight of hand trick of taking from one pocket and putting it in the other pocket. Good as gold until default day arrives.