MySpace just blew it. AOL was a safe walled garden for people who didn't know what the internet was. As the internet outside AOL increased in value, people had less use for their walled garden.
The same thing could happen to facebook if the rest of the internet gets more inherently social.
s/Facebook Fan page/AOL Keyword/
s/Facebook email/AOL email/
s/Facebook Games/AOL Games/Why wouldn't it be shocking? They've shown that they can ship and see through to maturity several revolutionary products. Stop thinking in the abstract: if a company employs teams who can build stuff, ship it, and bring it to maturity, that's what they'll keep doing. The tech industry isn't exactly lacking in revolutionary ideas to implement.
But, the chances are probably lower than Facebook failing. The chances of Google failing are much lower than the chances of Facebook growing its revenues (or prospects) 5X.
Price (ie working for shares) is an estimation of probability. I'm not saying Facebook is worthless. It's worth a lot. The reason Facebook's valuation isn't $100b already is because the road from here to there is an uncertain one. The two factors I would consider to be important are: (a) Facebook's revenue model is relatively immature.(b) Facebook might be a trend.
*I think Facebook might even have the potential rival Google in advertising. I'm not a Facebook pessimist.
The possibility of Facebook cratering is far greater than the possibility that Google will crater, but the possibility of Facebook reaching Google-like revenues is much more likely than the possibility that Google will double their revenue over a similar time period.
Considering that's 2-3x their current revenues can you understand the skepticism?