Here's the thing:

If you have a potential patent claim against Facebook (or any other large entity) you know that they will, as a matter of course, dig through their enormous stockpile of patents looking for some counter-claims to make the second you file. Given the size of that stockpile, they're probably going to end up with a sizeable stack, so you need to engage in some serious due diligence before filing to evaluate the strength of your claims and the strength of their likely claims.

All the React patent grant means is that, IF this happens, they can add any React-based patents to the pile, if any exist. I rather suspect that there are no React-based patents, but it doesn't really matter if they are, because statistically speaking, it's not going to materially change the size of the stack. (Not using React entirely also won't materially change the size of the stack; any hypothetical React patents probably apply to whatever hypothetical non-React tech you used instead.)

> it also discourage you from exercising any patent you have against Facebook

No, what discourages you from exercising a patent against Facebook is that they're Facebook, and they have vastly more money, lawyers, and patents than you do. The potential React patents are the least of your worries.

If you have a patent against Facebook, let's face it, that patent claim is going to be bigger than React ever can be. The cost of switching out a JS framework would be well worth the benefits of taking a legitimate patent claim on FB.