Another obstacle I think Facebook will eventually run into is the fact that social graphs, to me at least, seem fairly static. I am not a celebrity, and for me, literally ALL of my Facebook friends are people I know in real life.

While it is interesting to learn new tidbits of information about them, I am also very aware of the fact that, for instance, none of them live in Mountain View. If I decided I wanted to move there, the first thing I would do is turn to Google to learn about real-estate, utilities, getting a driver's license, etc. I don't quite see how placing the constraints of a social graph on top of a search for unknown information will make that search more useful. When stepping into uncharted territory, I don't want artificial limitations on what I find.