By using reactjs you are supporting their R&D. It improves their attractiveness for top researchers.

What if you use a website that uses reactjs? How culpable are you?

Less culpable.

Thank you for making value judgements on our behalf.

Well, in this case "more culpable" doesn't make much sense, or do you think otherwise? Perhaps equally culpable would be an option.

Not sure what point you are trying to make, though.

Didn't they try to push some rather questionable licensing terms on react or do I confuse it with a different open source library? If it was then its users almost became another victim of facebook trying out where it can get away with abusing its position.

They did have a license with a clause allowing them to sue or restrict access to it legally, but they changed it to a more permissive one. Also, even if they did try to do anything, no one was stopping you from replacing react with inferno or preact, they have compatible APIs.