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.
Completely agree. Facebook does something that is easily replicable, they've just managed to market themselves better than anybody else. But even that is starting to fail them as people are beginning to turn against the behemoth that is Facebook. Meanwhile, Google has amassed so much positive public relations with their self driving cars, fiber optic work, and being against SOPA/PIPA, that it's not even funny. Combine that with the practical and diverse nature of everything they have a part in, and you have a very, very solid company. Apple will probably always be Apple, they too rely heavily on marketing, which will run out at some point. They will also have a bit of a tough time as more people switch operating systems and move to Linux (I hope this happens).
I agree that Facebook will be the first to lose dominance. When you don't build something material, the reputation you have and the service you provide is doubly important. Going public created a huge appetite for profit, and there are so many ways that satisfying that appetite runs counter to their reputation and the quality of their service. It's already become a cliché to hate FB but use it because everyone is there.