Facebook was a fairly static site back then, but small is a relative term here. I don't mean to say it was only meant to do that. Just that the origins of React are a far cry from the complexity of SPA frameworks like Next JS. And it can still be used as quite a simple drop in component system. Nothing is stopping you from doing that. Which is why people often say React is not a framework but a library, because it's true.
https://legacy.reactjs.org/blog/2013/06/05/why-react.html