Keep in mind that all those page views and all that time spent on the site costs Facebook money. It's not like outdoor advertising where the cost is set up front (then amortized over the life of the sign, device or property lease) and the marginal cost for additional viewers is zero.
Another thing to keep in mind is that there are competitors constantly targeting Facebook. But not just targeting them on price, targeting the features of their site that keep people there for 30 minutes every day. This means that Facebook has to spend money to "improve". Again, compare this to outdoor advertising where the competition is largely just a matter of bidding on space every so often.
I, like you, have no idea what Facebook is worth or how successful their business model will turn out to be. However, I think it's important to keep in mind that turning page views (or minutes spent on the site, whatever) into money really isn't a fully solved problem, at least not in the general case. Facebook definitely has their work cut out for them I think.
Also, keep in mind that Facebook has attracted some section of the "best and the brightest". As far as I can tell, they do a really, really good keeping Facebook ahead of the curse; fun and ahead of potential competitors.
The thing is that since these people are the best, one might logically expect that they want compensation in some form, possibly only the form of knowing they're working on the leading edge or possibly in the form of lots of money. So Facebook's ongoing operations to an extent depend on project projections coming true. If they become like Yahoo, they'll bleed people like Yahoo and make horrible missteps like the way Yahoo rolled-out Axis (an idea that appeals to me but which was apparently utterly fumbled).
It's hard to imagine Facebook going the way of Yahoo. Facebook has a relentless focus on their product and continuing to ship and push the envelope. I don't know too much detail of Yahoo history, but it seems like they took their eye of the ball early and started spreading themselves really thin across a ton of disparate products and acquisitions.
I don't see anybody blindsiding Facebook on social the way Yahoo was blindsided by search. The bigger risk seems to be that there is simply no pot of gold at the end of the rainbow to justify a 100B valuation IPO valuation, thus souring everyone on FB and social before we see what the true potential was.
Another thing to keep in mind is that there are competitors constantly targeting Facebook. But not just targeting them on price, targeting the features of their site that keep people there for 30 minutes every day. This means that Facebook has to spend money to "improve". Again, compare this to outdoor advertising where the competition is largely just a matter of bidding on space every so often.
I, like you, have no idea what Facebook is worth or how successful their business model will turn out to be. However, I think it's important to keep in mind that turning page views (or minutes spent on the site, whatever) into money really isn't a fully solved problem, at least not in the general case. Facebook definitely has their work cut out for them I think.