It's really not a glorified phonebook, as much as a fairly public little black book. Phonebooks can actually be almost decent places to put ads.

The way I see facebook being, is that it's an 'end of the road' site for me. Meaning that asides from my friends posting outbound links, I'm not going to go anywhere from there. The opposite of Google. I go to Google to go elsewhere. 99% of time when I leave Facebook it's just by closing the tab.