Within one week of its launch in October, photo sharing app Instagram had 100,000 signups. Seven months later, it had 4.25 million users. Online photo sharing has long-been popular: Picasa launched in 2002; Flickr was founded in 2004, and Facebook was home to about 60 billion photos by the end of 2010. But mobile applications have suddenly exploded in popularity. Photo shooting and editing apps li
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