Sure, we're putting more in now than is being paid out.
Sure the surplus has to be invested somehow, and certainly US Treasuries are a safe place to park it.
But the gov't is applying accounting standards which would get a CFO a long vacation behind bars. They are double booking the revenue -- it appears as an asset in the 'trust fund' and as general revenue to offset the current deficit. This is how the national debt managed to rise every year of Clinton's administration, 'surplus' or no. But as every embezzler knows, the juggling act can't go on forever.
In about 15 years, a whole lot of chickens will be coming home to roost.