I guess Biz Stone came directly from Google to found Twitter.
Given Google's size and the quality of the people who go in there, I'd have expected dozens if not hundreds of companies to be spawned by ex-Googlers.
I often wonder if certain companies simply produce more founders because of culture, money or simply bias in who they hire. PayPal is the classic example of a company that pumped out a good number of successful founders, but I can't identify any particular characteristic of that particular company that might have caused it.
I'd love to see some analytics about where the most successful or prolific founders come from.