This is an open discussion, many of you are Social Media connoisseurs, I'm interested to know, from your perspective, what are the common features that make a Social Network fly?
Perhaps it is a great niche idea? Or maybe they were the first to implement groovy functionality? Or perhaps they just got lucky? Or maybe it is a symphony of multiple factors. There's a few experts here, I'd love to know your thoughts, feel free to share links to example sites as well.
My ideas are in the comments!