Pre Mayer Yahoo did sue Facebook over patents. They settled. As far as I could see the fact that the only big patent fight fb has been in ended with a company with a boatload of lawyers being cool with these terms should bode well for anyone else. If you have patents they shouldn't rely explicitly on open source library x y z, but I am not a lawyer either.
> Mutually assured destruction
Wouldn't that apply to Google as well?
I'm not really sure what Facebook thinks they are gaining with this clause. They aren't going to stop patent trolls, because they are non-practicing entities. Maybe they want to guard against someone making improvements to React and then suing Facebook if they do something similar later.