Facebook is an advertisers dream. I don't think it could be any more perfect from an advertisers perspective, and I'm sure facebook charges a premium for this kind of consumer data. However, targeted advertising only works when there are targets...what happens when/if facebook goes the way of Myspace and Live Journal (not that LJ was a behemoth like facebook or myspace).
It seems like myspace was king for a few years, then tanked and facebook filled the void. I'm not sure what caused myspace's drop in popularity, but one has to wonder if facebook will see the same thing. Seeing as how "every one" read - everyone's parents/grandparents, are on facebook, you have to wonder if the people who fueled facebook's uprising (the college age crowd) will packup and move to the next big social net where nobody's parents are there to see the kind of debauchery in which they take part.
Then facebook is left with millions of square feet of data center space filled with thousands of idle servers. Cell phone company stop bundling facebook apps out of the box, and companies and news stations no longer update their facebook pages, because they too have moved on to follow the masses to next big thing.
If I were to invest in facebook, I would ride that wave for a very short time. Maybe I'm wrong, the world is much different than it was in 1999.