The US is "a Ponzi scheme that works"

Coming back to the US is always wonderful for one reason in particular: I am so glad that this is my home. 
The Economist does a fantastic job of explaining why the US is a fantastic home to immigrants.  Read it before it disappears from freeloaders like me :-)
Excerpt, which is also the concluding paragraph:

The stakes are high. Immigration keeps America young, strong and growing. “The populations of Europe, Russia and Japan are declining, and those of China and India are levelling off. The United States alone among great powers will be increasing its share of world population over time,” predicts Michael Lind of the New America Foundation, a think-tank. By 2050, there could be 500m Americans; by 2100, a billion. That means America could remain the pre-eminent nation for longer than many people expect. “Relying on the import of money, workers, and brains,” writes Mr Lind, America is “a Ponzi scheme that works.”