Fear and Speculation Drove Facebook's Instagram Buy

Third in a five-part series. Sometime in August, Facebook will top 1 billion users. Every few months, Twitter floats a press release that it has added another 100 million users. In quarterly earnings calls during the past year, Google has had to sheepishly admit that it its Google+ social network isn't growing as fast as hoped. Pinterest is the hottest new social network, in large part because it...