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I haven't heard of Facebook having a reputation for abruptly shutting platforms down. On the contrary, I believe its widely accepted that they handled the shutdown of Parse very gracefully. Year+ notice, with significant open sourcing and community efforts to maintain continuity.

At this point I think React has wide enough adoption with other large companies that it could easily survive even if Facebook abandoned it.

Well React and React Native are two different things, just because they move away from it for their apps doesn't mean they're going to move away from it on the web.