If initiating a patent war with Facebook is a foreseeable business outcome for you, then you probably shouldn't use React. I remain convinced that logic applies to only a tiny minority of companies.
My reading of the license is that you lose Facebook patent grant even if your lawsuit was a response to Facebook's.
I am not a fan of (software) patents, but license restrictions are not limited to them either. So what is your recourse if Facebook decides to infringe yours?
Assuming this is true (IANAL), would you still feel as comfortable advising companies to use React?
I don't begrudge Facebook for their license for the same reasons that Matt stated, but I am highlighting this issue to my clients and would not recommend React's use because of it.