It's important to remember that Zuckerberg kept saying 'I don't know', because he was thinking out loud, not speaking from a script. Zuckerberg said he would s

It's important to remember that Zuckerberg kept saying 'I don't know', because he was thinking out loud, not speaking from a script. Zuckerberg said he would stay in Boston or move to New York, in consideration with moving to Silicon Valley, if he was starting Facebook right now, and knew what he knows already. He would start a company in a location where it was commonplace to build sustainable businesses, with employees who grow within a company. He admitted that six or seven years ago, when he didn't know anything or anyone, he could not have done Facebook without being in Silicon Valley. My thought is that if a big consumer-facing/social tech company like Facebook was successful in Boston (now or in the future), many more companies like Zynga and other startups would be based in Boston as well. Facebook's and Google's success (and their willingness to buy startups that may or may not have a viable business at the time) causes a side effect where highly paid employees save up money and quit these and similar companies in order to create startups just a few miles away. After all, Zuckerberg also stated he believed that the next wave of applications will have to integrate with social networks or else will die. If this is true, he must expect that a ton of startups would be started in garages and incubators all around where such social networks are based (Facebook and Twitter, which happen to be in Palo Alto and San Francisco.) It would probably be more accurate to believe that Zuckerberg means that Silicon Valley is the best place to start a company, but he wishes it had a startup culture that he believes other cities like Boston and New York have right now (while they don't have very many giant web properties headquartered there). He doesn't realize that a Facebook in Boston would mean that 1) investors were more likely to invest in startups than they really had been and 2) the built-to-flip companies of ex-Google and ex-Facebook employees (and those of other companies) would now be based in Boston or New York, both factors changing the startup culture of that city.