I'm no expert when it comes to sales & marketing, however, it's hard to dispute the success of other social media platforms like facebook and twitter, neither of which charge a registration fee.

IMHO a fee to join just hinders adoption and it puzzles me to see you propose it. It would make sense to charge a reg fee once you reach a certain stage of growth, but to require it before then seems counter-intuitive. Then there is the psychology, "we have great shit, and we're proud of it. Come join us so you can be the first of your friends who found this wonderful, shiny new thingy". It takes a lot to reach that tipping point. Just don't see the psychology of your approach.

Care to elaborate on why you think it's necessary or why it won't hinder adoption?