The author mentions abandoning MySpace moving into highschool like the subject graduates past Tumblr.
It's symbolic of the maturing nature of the individual. In the future perhaps social networks will find a way to iterate their brand with each generation to prevent losing users to another new service.
This exact issue seems to apply to online gaming as well, so this isn't some new discovery of human behavior. There are some 'big' games (warcraft, cod) that retain their users even after a decade - but they do this by reinventing themselves and recreating their products constantly. I think social media and gaming are similar enough for a lot of these behavior traits to apply to social media also - which means the correct approach is to redesign/improve core parts of your social platform at least once a year to keep it relevant.
People just want a consistent interface to communicate. There's no UX reason to separate email, facebook, instachat twumblr or whatever from each other.
Yes there are different reasons for communicating but they can all function under one roof. The next big trend of sending pokes with 12 second color corrected videos that get deleted after watching can live inside Facebook, G+, or TwEmail 3.0
But then, maybe you're right - maybe we should look forward to some incredibly sophisticated, calculating companies in the future.