Sure Facebook is a decently designed site that makes sharing your photos, videos, comments, links, etc. easy. But that’s not hard to replicate (the backend scaling parts are hard, but invisible to the end user). When people find the new social network that all the cool kids are using, they’ll flock to it and abandon Facebook in an instant, leaving shareholders dazed and confused. I doubt Facebook will die as quick & horrible a death as MySpace (in addition to being more fad-driven, MySpace was also centered around a fad-ish industry and had horrible usability), but it will die a similar death.
Google on the other hand has developed revolutionary technologies that other companies find nearly impossible to find the talent & resources necessary to replicate and improve upon. They provide a real, unique, non-fad value proposition to the end user. Same with Amazon, Apple and many other established tech companies.
Facebook investors' problem is, will they be able to get a return on their money before a new social network becomes the trendier place to be.