I don't think Google will go anywhere anytime soon. They have there hands in soooo many things. Probably be around my entire lifetime. They are too big to be baught, and too diversified to fail. They have the search engine, a monopoly on online advertising structures, email, google docs, Google+ which will grow as the afformentioned Facebook fails. And that's JUST their web stuff. In addition to that they have the self driving car program, they have the leading phone OS, are working on a Desktop OS, are working on hardware, have hardware hosting technologies coming, Fiber ISP services coming, VOIP phone service coming, etc. They are spreading out across all kinds of technologies and sectors, that unless we run out of electricity I don't see them going anywhere.
Facebook is only still around because that's where everyone is at. But eventually everyone will go somewhere else. They will last only as long as the fad lasts, and eventually people will leave for another service, or they will get bought out by someone bigger... maybe Google. Lol. Their stock price reflects that they can't capitalize on revenue and aren't going to hold those inflated share prices aloft.
Apple? That's anyones guess. Without Jobs around anymore, it's hard to say where that company will go. If they don't diversify I think they might get left behind. Right now there market is flashy gadgets, and premium versions of things that tons of other companies makes. What I mean is, they aren't the first to make anything, but they make a very slick, streamlined, and luxury version of it. The iPad, the iPhone, etc. But it's mostly marketing and their OS/iOS that is keeping them afloat. There really isn't anything stunning they are doing that you can't do with another cheaper product. Not trying to start a flame ware, but that's just the way it is. Apple products charge a premium for their gadgets. I see them being around for awhile as long as somone else doesn't go after their market share, which really isn't happening right now. But who knows, without Jobs that company might run itself into the ground.