The story of Twitter's success began on 21 March, 2006. When the US social networking service floated its stock in 2013, many observers thought it could be bigger than Facebook. It hasn't been. Twitter is a victim of Silicon Valley's unwritten rule: anything short of total market domination counts as a flop. But Twitter can boast an impressive 320 million active users and over 500 million passive ones. For the past two years, revenues have been soaring. But the service has yet to generate a profit. So the big question on Twitter’s tenth birthday is: where to now?