What motivates users to add/create contents in your web service (UGC)?

There are a lot of CGM out there, including Dribbble, Yelp, Facebook, Hacker News, and others. What I am curious about CGM is that what motivates users to share/add/create/etc. contents within the web service. CGM usually consists of content generator and content consumer, and it is a Chicken-and-Egg problem to start a new CGM. You need contents to bring in traffic, but you also need traffic for content makers to keep adding new contents. What are content makers' motivation? Why would they add contents to your CGM? What are some of strategies to motivates users to create contents? It would be great if you can share your thoughts and insights :)!