There was no sign of an exodus. That’s ludicrously hyperbolic.
Projects that I work(ed) on that use React have no plans to switch away and aren't represented in that 25%.
25% of sites on the Internet run WordPress. WordPress was rewriting its editor in React, but decided to switch away due to BSD+Patents.
FWIW, why we prefer Apache 2.0: https://www.cncf.io/blog/2017/02/01/cncf-recommends-aslv2/
This Facebook post gives us hope for GraphQL moving away from BSD+, as it is currently in our stack, but without doubt we are considering a move away due to uncertainty.
Evidently FB saw enough people leaving to prompt this change. No doubt they were also afraid of an exodus of their internal developers on these projects. High-profile developers will move on to companies where they know they'll have greater impact if that's seen to be at risk.
Those were the only two.
But it did matter.
It set a strong precedent.
Some 28.7% of sites use Wordpress.* Curiously, I've seen the 19% number being cited as 19% of all new websites use Wordpress. I'm not sure how 19% becomes 28.7%, but it may just be that sites using WP stick around longer.
* https://w3techs.com/technologies/details/cm-wordpress/all/al...
Even being on the back-foot when having to defend your patents, by refactoring React out of your application, was apparently damning enough for people to migrate.
If you're ok with trusting a direct competitor not to misbehave, then sure, use React under those terms while competing with FB. But large companies consider their patent portfolio to be a significant strategic asset, so it shouldn't be surprising that their legal counsel advises against accepting such a clause.