As long as Debt increases slower than inflation your paying it down.

What cost $5605.5 in 1999 would cost $5796.09 in 2000. http://www.westegg.com/inflation/infl.cgi and 5769.9 is less than $5796.09.

Edit: I have seen the same argument vs infrastructure or GPD. But I think inflation is probably the safest measurement, because GDP can drop.