Are you rewarding good behavior, or just any behavior?

I was recently chatting with an entrepreneur who was considering building a leader-board for his social web app. The purpose of the leader-board, he told me, was to display those users of the app who were most active, with the idea that this would incentivize them (and those not on the leader-board) to participate more. With the lure of climbing the leader-board and sitting at the top, users would...