Give people something cool to do and a way to talk to each other, moderate a little bit, and your job is done. Games like Eve Online or WoW have developed entire economies on top of what's basically a message board. MetaFilter, Reddit, LiveJournal and SA all started with a couple of buttons and a textfield and have produced some fascinating subcultures. And maybe the purest (!) example is 4chan, a Lord of the Flies community that invents all the stuff you end up sharing elsewhere: image macros, copypasta, rage comics, the lolrus. The data model for 4chan is three fields long - image, timestamp, text. Now tell me one bit of original culture that's ever come out of Facebook.
Right now the social networking sites occupy a similar position to CompuServe, Prodigy, or AOL in the mid 90's. At that time each company was trying to figure out how to become a mass-market gateway to the Internet. Looking back now, their early attempts look ridiculous and doomed to failure, for we have seen the Web, and we have tasted of the blogroll and the lolcat and found that they were good.