When Facebook bought Instagram for $1 billion in 2012, critics of the social network said the deal was another sign of Silicon Valley overreach. Instagram, a photo sharing app with just 30 million users at the time, had no revenues. Facebook, still a private company at the time, was making its biggest and riskiest deal yet. And few start-ups, let alone apps, were valued at $1 billion or more. Less than three years later, Facebook looks prescient. Silicon Valley is awash in billion-dollar start-ups, and Instagram is valued at $35 billion.