Until they don't. The past is an unreliable predictor of the future[1].

The only people who care about being on a fashionable network are young people, and Facebook solves that problem by buying whatever platform young people are using.

1. https://en.wikipedia.org/wiki/Problem_of_induction

rtruin's argument is an intuitive assumption that the same causal factors for other like companies are also present in Facebook, which sets the frame for discussion -- what factors lead to the diminishment of other networks, and why is Facebook resilient against those factors?

You may be identifying that young people set trends and thus the growth rate of different social networks, and you may be arguing that Facebook is resilient because they can buy out companies. But so can Microsoft, Google, Apple, MySpace, or Yahoo. That power is not unique.

One might also say that young people who join some social network like the peculiarities of that network, and so it behooves the parent company to keep that acquisition fairly independent in brand and experience. That means the existing strength of your platform might be a non-factor to the future success of WhatsApp or Instagram.

Which is why they keep buying the latest cool