Bullshit article premise
These are just large sites. They're not the internet, and the internet is not "centralized" around them.
Google's a big search engine because it's superseded all the crap ones before it, and because Bing, etc just aren't very good.
If Google went bankrupt tomorrow (which isn't going to happen) then people would drop back to Bing and whatever else, until someone else replicated Google's success.
It's the same with Amazon and Facebook. They're big and popular because they happen to be the "best" at what they do at the moment, where "best' is defined as "not as shitty as the rest" -- if people get pissed off with Amazon's customer service, treatment of their employees, or bogus reviews, or what-have-you, then they'll leave and go somewhere better, if that exists.
For example, I use Amazon a lot as a "shopping search engine" then I find the actual manufacturer/vendor and buy from them.
A lot of the time I can't do that because the manufacturer has decided they don't want to actually sell their stuff "to the little people" and tries to pawn me off on some crappy distributor, and other times they just don't have a working site, in which case I fall back to Amazon.
This is why Amazon makes lots of money, because other people are incompetent at selling their stuff. They put all sorts of obstacles in the way of you giving them money.
They're big and they supply an important service, but that doesn't make them "the internet"