Facebook has huge promise, but I think it is still too young to tell if it has legs on the order of a century. On the other hand, Google has already shown its ability to hop platforms again and again (Search to E-mail, Web to Mobile, Software to Automotive), and their head-start in artificial intelligence will make them an important force for at least two decades. I would bet Google is the company of this generation…
I agree about Google, but I don't think Facebook is even remotely in the same class. For all I know it could be gone in a couple of years. Apple and Amazon seem like two much more likely candidates, along with Google.
Assuming that the core priority is to maintain and optimise their core business to a certain point of stability, they can then look at branching out into other areas that present attractive opportunities.
Of course Google has developed a very specific method to encourage this kind of behaviour (80/20) and Amazon has yet again quite a different process, so it would be very interesting to see how Facebook will go about this in the next few years.