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.
Anyhow, it could well the world's greatest computer engineers, but I doubt even that will allow it to survive the shifting sands of both fashion and technology over the next hundred years. Its not that facebook is bad, just that I very much doubt it'll be plugged into the technology that my children use to communicate in high school, twenty to thirty years from now. It may be telepathy.
True, but staying relevant in the field is not an engineering problem, it is a managerial problem. Microsoft had (and still has) a lot of talented engineers, yet they missed out on some great opportunities because of poor managerial decisions. That's why in 2011 they are not as relevant as they were in the 90s.