Isn't this just another way of saying survivor bias? They are big because they survived and out-competed the competition, no? There were and are other operating systems, other search engines, other social networks. And sometimes even having the same things said about them... Think MySpace before Facebook. Until they got conquered.
These giants are huge because the internet -- to some approximation -- has no geographic boundaries. It's much easier to consolidate and hold power when you don't have to literally expand to every corner of the world where people physically live to capture them. Amazon and Facebook can acquire entire towns with nothing but a few more servers in the rack.