Pinboard’s maestro says it best:
The funny thing is, no one’s really hiding the secret of how to make awesome online communities. 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.