Will Facebook have access to all technology that I’ll develop using React?
No. It’s a myth. Facebook has this clause that if you develop something in React you can never sue Facebook of infringing that same thing in future. While that looks a big deal take a second and think about it. Only thing which you can develop through React is web application and that can be and will be copied if Facebook wants to. Facebook copied almost the entire idea of SnapChat and they couldn’t do anything about it.
You don’t develop your IP ie your Intellectual Property in a JS framework. That code is anyway residing on the client side almost all its useful life.
Let this sink in.