> the single most important factor that determines startup success is hiring quality talent, specifically engineers and scientists. High quality engineers and scientists are currently a small percentage of the entire world's population. They are attracted to areas where they can find jobs, and usually stick around universities.

Another thing is the speed at which a company can hire high quality engineers. Facebook now has 25 employees for every employee it had eight years ago. For a company taking off like a rocket, there is only one place where you can hire that many good engineers in that amount of time - San Francisco.