Talk about Web 2.0 and people instantly think of Facebook, MySpace, YouTube, Wikipedia: communities rising out of user-generated content. But if you are talking about user-generated content and communities then for me the defining site is still eBay.
I started using eBay in anger a while ago. I am blown away with the trading. Obviously I have a some views about shortcomings in their auction design (e.g. a seller with a feedback score of 0 can only send a message to one bidder a day in an auction she is running) but that’s another story.
What interested me most on eBay is the community. People are trading, exchanging, doing real tangible stuff on a day to day basis. And communicating with each other, if only through their feedbacks. On top of this, you can often see a history of what people have bought and sold which tells you an incredible amount about a person. Now, that is more meaningful (and probably monetisable) information about someone than you will find in any number of writings on a Facebook wall.