It is a big shell game, but it seems to be going along just fine.
It looks like about a 1/3 of those $100s are owed to US citizens, with a 1/3 being owed to the governement itself, mainly through them taking your social security taxes and buying US bonds with them. The last 1/3 of the $16 trillion being owed to foriegn entities.
http://en.wikipedia.org/wiki/United_States_public_debt#Debt_holdingsIf we all educate our citizens and quit killing each other we can keep up this giant ponzi scheme untill at least the first few Dyson spheres.