Imagine a social network where users have invested so much social capital in putting up data about themselves that it is impossible to imagine them leaving. Moving to a new site would be an enormous risk for users because you would lose your network of friends. The network’s entire existence, the theory goes, is secured by these barriers to starting afresh at a new outlet. This was how the Guardian described Myspace in 2007, when the early social network had 150 million global users, a number so large it was considered improbable that they would ever move elsewhere. (In the...