Well, I guess it depends on what you mean 'almost every single large tech company', but React is used by Facebook (of course), Apple, Twitter, Yahoo, Netflix, Amazon, just to name a few.
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.
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
...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).
