The continuing value added to Facebook's bottom line by React is in granting influence far outside of their natural scope. Could they counter Mozilla's move by introducing new React "features" that Mozilla simply "can't support" yet, even though they "really super duper hope the new featureset can come to Gecko-based browsers next year"? Even more insidiously, could there possibly be some little snafus in their QA process that let bugs that affect Gecko-based browsers go unnoticed?
That's their leverage, that's their retaliation against Apple and Mozilla's moves to stymie their flow of user behavioral data. But, I'm sure a company whose revenue stream is wholly predicated upon reselling user identity would never do something "shady" like this to prevent competitors from blocking their access to the data that forms the lifeblood of their product, right?
... Right?