What actual activity behavior? Care to back up this statement with something? Anything? A ridiculous illustration of confirmation bias.
Like I said, several countries have higher net migration rates. A few of those countries also score higher in income distribution, social mobility, happiness, health and life expectancy (again, so far as you trust these measures, some of which are, admittedly, highly subjective). Strangely enough, Canada is one of the countries that has the US beat on ALL of the listed stats, although as I've stated they lag behind the US in GDP per capita by about 10% (I'd guess this gap narrows with the continuing economic troubles in the US...Canada hasn't been hit nearly as hard).
There is little basis to say that the US is currently a country of unprecedented opportunity. If you care to explain why you've stated as much, I'm open to changing my mind.