Instagram is one of tech's great success stories. The network has seen incredible growth over the past couple of years to become a platform with 1 billion users. So this week, when founders Kevin Systrom and Mike Krieger announced they were leaving the company, it came as a shock to many, even Facebook itself — Instagram's parent company was caught flat-footed without a prepared statement until the next day. But perhaps the news shouldn't have come as a complete surprise.