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I know atleast one of these companies (I can't speak for the rest) had a pre-existing bi-lateral patent agreement with Facebook which rendered the react patent clause irrelevant, allowing them to use react without concern. Without that existing agreement this would not have been the case.

I would certainly concede that. And I'm not trying to rain on React's parade here. I'm just saying that HN is particularly pro-React as a community.

you have only named a few though; 'almost every single large tech company' requires a particular definition of large tech company or a lot more than a few to be a useful(ly true) statement

Take a look here:

https://github.com/facebook/react/wiki/sites-using-react

...that's obviously not complete, either. If you think of a large tech company the odds are they are using react somewhere for something, be it public facing or internal tooling. Many of the companies that aren't actively using React do so because they have their own JS stacks to dogfood (like Google and Angular).

Google hardly dogfoods Angular. They use Closure for most of their apps. SPF for YouTube. Polymer in some places. GWT in many others.