We are effectively saying that each user should generate an average comment score of 1 point per day to break even. Anything you make beyond 1 point is considered an excess return for the day. We then simply take the Sharpe ratio of the average daily excess returns. The resulting metric ensures that users are incentivized to make consistent, high-quality submissions and punishes one hit wonders and those who take a spray-and-pray approach.
In my opinion, we want people to keep quiet until they have something meaningful to contribute to the conversation. The idea that making any post is a "return on investment" is nonsense.