[countryslicker]

Why don't FB/Google/Apple/etc expand to Auburn, CA? Or Lincoln, CA? or Lincoln, NE?

I really don't think that putting the next 5000 employees next to the last 10000 employees improves productivity. Spread out. Spread the wealth and the pain. Make yourself less vulnerable to a single point of failure.

[ThrustVectoring]

It's not just about productivity from the employer's side, but also about the negotiating power from employees. When FB/Google/Apple are all in the SF Bay Area, one of their elite employees can more easily get competitive offers from the others.