The great majority of native-born Israelis who leave for abroad are back within years. They often bring with them new skills and attract hi-tech companies, who saw what they could do. That's how Israel became a hi-tech hub.
This circular migration is often a boon, since many Israelis get valuable educational and work experience abroad before returning.
The only reason there are so many Israelis abroad is because a great percentage of non-Jewish Russians (and admittedly, some legitimate Russian Jews too), stayed in Israel for a bit, took the immigrant benefits, and left for abroad. In 2010, one out of every two Israelis applying for a Green Card was Russian. And it seems the great majority of Russians who leave are not Jewish. Then, there are many Jews who moved to Israel, lived there for a while, and left. And finally, the children of yordim are counted as Israelis up to the first generation, even if they were never in Israel or speak Hebrew.
On a side note, Israel is actually retaining more of it's educated population than many other developed countries, including Switzerland.
It's the Palestinians who are experiencing a brain drain. Every year, up to 20,000 leave the West Bank, mostly young and educated.