"If you use React, doesn't it basically mean that you de facto license all your patents to Facebook whereas Facebook licenses those required strictly for React?"
No.
The only patent grants you give are through CLA's.
Otherwise, i'm not sure i follow the concern?
Say you use React and own patents. If you ever sue someone over patents, it may terminate your React license in the case where the defendant uses React. You may not even know it and keep using React happily after termination. Later, you can't sue Facebook without them countering that your use of React has been unlicensed since termination. Is this a valid concern?
While rereading the grant, I even wonder: (1) does the termination in the grant mean termination of the copyright license as well? (2) asserting any patent against "any party relating to the Software" could include end users since they receive the same license and grant.
No. The only patent grants you give are through CLA's. Otherwise, i'm not sure i follow the concern?