US debt to foreign investors has doubled in volume
since the start of 2009 (which was the height of the financial crisis). This indicates that despite the fact that the crisis occurred and was initiated in the US, the demand for their debt not only did not vanish - it actually spiked. In mu humble opinion, it is a very strong indicator of the actual strength of US economy.
Please note, that the level of debt that US actually owns to the rest of the world is much lower than 100% of GDP. Actually, as long as the debt COST is affordable (and it is affordable now) - debt is not much of a problem.
For more detailed info (very interesting stuff), please see: https://www.treasury.gov/ticdata/Publish/mfh.txt