> So many big companies with tons of lawyers use React... surely if they do it it's fine.

At best, that demonstrates that its fine...if you have lots of lawyers.

You mean unlike every company that is behind any other project you are relying on that doesn't come with an explicit patent grant?

In other words you have no MIT or BSD licensed dependencies, or in fact no open source dependencies that don't either come with an explicit non-revocable patent grant and aren't licensed under Apache 2.0, GPLv3 or MPL 2.0 (all of which by the way still terminate the patent grant if you sue over infringement of patents within the licensed code itself).

Lucky you?

How, exactly? My company doesn't have any patents; what legal leverage does Facebook have over us? (Trick question, the answer is "zero".)