Want to grow? Throw a Party For Your Best Users!
Inc.com has an interesting read on yelp.com's rise to success. One of the most interesting observations is yelp's exponential growth started after they began inviting their top reviewer's to a frequent open bar party. They encouraged the expansion of their content, and thus relevancy, by responding to the most basic uniting factor of their user base: the desire to go out and be social.