Facebook itself won't be meaningfully "deleted" for at
least five years. It simply has too much money: even if its main product went kaput tomorrow (which it won't), it would find some other way to stay alive. That would almost certainly be via an acquisition spree, in which it would go to great lengths to tie its targets' platforms into facebook.com, which is based on React (AFAIK). So the answer is: nothing.
That said, what if, as a thought experiment, Facebook imploded tomorrow? It reminds me of the "what if the Sun disappeared" scenario -- it's not that it's _unlikely_; it's that it would require new physics to even get you there. So speculation is somewhat idle. That said, here's what I think would happen:
1) The core contributors would be snapped up by some open source-adjacent organization, like Mozilla, and given free reign to work on React in the majority of their time.
2) Some sort of new, independent governance would be formed. IME, new web projects seem to be more ad-hoc in this respect than, say, GNU brethren.
3) Development velocity would undoubtedly slow down. This happens naturally as projects mature, though, so let's not mourn the inevitable process of nature.
4) Some idealistic fork will pick up a bit of steam, and the sister projects will steal from each other liberally. Like bacteria, politicians (zing), or anything else that competes.
5) Something else will rise to take React's place.
But again, it doesn't matter, because Facebook won't disappear on a time scale that matters relative to the velocity of JS development.