According to OECD brain drain report in 2007 (http://bit.ly/b9UNrH%20)
If you specify college graduate brain drain, US is again one of the lowest brain drain rate (less than 2% ---- http://jobs.renego.de/ein
Having said that, please take a look at Gallup's Potential Net Brain Gain Index. US is one of the top desired destination countries in the world for potential migrants -- is poised to experience higher youth gains than education gains. The Potential Net Brain Gain score of 12% in the United States, however, is relatively small compared with its Potential Net Migration Index, which indicates that this country would likely attract more people but not necessarily more educated people. In other words, US needs to compete against Australia, NZ, Singapore and Northern European countries to attract educated immigrants. http://bit.ly/cGWkfo
Lastly, over last few years, due to great recession, so many educated H1-B holders had to leave US. It is very worrisome trend. http://bit.ly/ennFoe I am not sure this is one time trend during recession or not. Since there is movement to grant US working permit for all of US college graduates with science degree, I hope that this law will pass and that it will stop US brain drain.
Hope it helps!