Joshua Gans has a theory:
The network effects by which users value a particular network increase based on the amount of sharing going on. One telephone is useless. But with many millions and a phone book, you're getting somewhere. Facebook's defaults nudge people towards sharing while Google's do not. Indeed, Google's entire attraction is its sales pitch, "Suppose you don't want to share something with all your friends, we make that easy." So it attracts users more inclined to limit sharing. Here's the business implication: in the end, sharing is how social networks will make money and Facebook has positioned itself to maximize both.
Previous Dish autopsy on the social network here.
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