...but so many smaller companies and startups use React. Personally all of my employments (five at this point) have used it and none had more than twenty programmers. Has it actually been an issue in real life?
Or FB sues you over literally any patent you happen to be infringing. They're a big company, I'm sure their patent war chest has a few zingers for cases like these.
Oh, and your scenario only works if you sue FB/Instagram over your patents first. They can't revoke the patent grant willy-nilly except in retaliation.
I should point out we don't even know whether Facebook even has any patents for anything React does. And even if they do we don't know whether there are other libraries that do the same things and therefore infringe on those patents, making you liable if you use them.
It's software patents you should be worried about, not open source projects that try to offer you even the tiniest amount of protection from them.
At best, that demonstrates that its fine...if you have lots of lawyers.