As I type these words, a mass exodus is underway from Twitter and Facebook. After decades of eye-popping growth, these social media sites are contracting at an alarming rate.

In some ways, this shouldn’t surprise us. All the social networks that preceded the current generation experienced this pattern: SixDegrees, Friendster, MySpace, and Bebo all exploded onto the scene. One day, they were sparsely populated fringe services, the next day, every­one you knew was using them and you had to sign up to stay in touch. Then, just as quickly, they imploded, turning into ghost towns, then punchlines, then forgotten ruins.