And I doubt it's a mass exodus either.
If you read "Surely you're joking Mr Feynman", the author clearly states that when making the bomb, they had a lot of fun. Lots of budgets. Only smart people. Working on the coolest projects. He even plays with the censorship and security practices.
They never really thought too hard about the ethic part of it: they need to end the war, and that's it. They really said "oh we fucked up" once the bomb exploded.
And we are talking about brilliant minds with very positive personalities.
While skilled, I doubt than more than a small fraction of FB work force is close to Feynman's IQ. And they have a very arrogant culture. Somehow I doubt FB is going to bleed talent anytime soon.