I’m not a lawyer, but from my perspective, that’s indeed a concern. And perhaps you could get sued just by using React. It could differ between jurisdictions as well.
A standard open source license with a patent grant, like the Apache license, would have been a lot clearer, but Facebook has so far refused to license React in that way.
A problematic patent grant was offered for earlier versions of React but that’s not the case anymore (and didn’t really fix the problem anyway).
Free Software is an established term, and React certainly is not Free Software, due to its patent. I very much doubt that React is Open Source either, for the same reason.
React is patented and Facebook is actively choosing not to offer a patent grant, so unfortunately that’s not the whole story.
A fork may not be an option. Perhaps a given organization may not even be allowed to use React, if Facebook decides against it for some reason. Jurisdictions can differ as well.
In my opinion, the best thing would be if Facebook simply made the terms clear by using the Apache license or similar. But hey, it’s Facebook, so I’m not expecting much…
A standard open source license with a patent grant, like the Apache license, would have been a lot clearer, but Facebook has so far refused to license React in that way.
A problematic patent grant was offered for earlier versions of React but that’s not the case anymore (and didn’t really fix the problem anyway).