Facebook will fail first, in my opinion. They're not very diverse.
In my life, it wouldn't effect me at all if Apple disappeared. But all biases aside, I think if Facebook ceased to exist, life would carry on almost exactly as it did before. If Apple or Google went away, there would be some more serious consequences.
I don't think Google or Apple will fail. I think Google has a solid long-term game, and while they have their hands in literally everything on the internet, they are getting into more "IRL" things, like self driving cars, being an ISP, etc. I think Apple will eventually end up like Microsoft, making products that are still relevant, but are no longer cool. Who will take their place is anyone's guess.