For me, this is one of the more "fun" things about the Bay Area, where people are at company A for a while then move on to company B, C, ... When I arrived it was the big semiconductor companies that had engineers going out and building new things, then folks from Sun, from Apple, when the first wave of people left Google they created a mini boomlet in new ideas, now Twitter. I expect former Facebook, LinkedIn, Netflix, and Palintir folks to part of the next cadre. Join a company, experience its growth, figure out how you can do it "better" or "different" and move to a new company to do that. The nerd equivalent of the circle of life.