Chris Willis has posted a presentation on the Hypergene media blog, that lists why some social media sites like Facebook are instant hits while others are not. He says that those who are successful have at least these 7 things in common:
They start with a compelling idea & simple solution.
They let people make their stuff better, more findable or entertaining.
They live by the Golden Rule – be nice to others.
They encourage lots of feedback.
They create “usable exhaust” – new things are created just by people doing stuff they want to do.
They let many groups form easily and quickly.
They recognize and encourage the good people in the network.
Further explanation can be found in this pdf, “We Media: How Audiences Are Changing The Future of News and Information”.