Though "Facebook will fail because pretty much all companies fail eventually" is a reasonable one.
No-one, looking at Apple in 1998, could have correctly predicted what they would be like a decade down the road. It's not unreasonable to suppose that guesses at what the big companies in the industry will look like a decade from now will often be wildly incorrect, no matter how obvious things seem.
No, I stand by my statement. I don't believe that Facebook will continually dominate the hearts and minds of the online world in the way it does now, in perpetuity. My reason given for failure was "complacency and failure to inovate" which I believe is the downfall of most great companies. The companies that I listed were examples of this phenomenon. You can really take this theory all the way back to the Dutch east India trading company if you want to. Evolve or die. In all probability, someday facebook will stop evolving.
Now I'm currently refering to facebook failing in the same way that "Microsoft and Yahoo" have failed. They still exist, are still (quite) profitable, but their relavency is fading.
As to apple. I've always wondered if the magic there will continue after Steve Jobs dies.
Apple's secret to ongoing growth and success is introducing a major new product category every few years. Six months takes nothing out of that--the long term visionary work Steve Jobs does can be missed for six months without much impact. Six years and Apple would be in decline.
No-one, looking at Apple in 1998, could have correctly predicted what they would be like a decade down the road. It's not unreasonable to suppose that guesses at what the big companies in the industry will look like a decade from now will often be wildly incorrect, no matter how obvious things seem.