This is an interesting point. I was thinking about my own thought process to some extent, which is: what is the probability that someone will fill this prompt? I prompt in the hope of it being filled, but I get your point, since just reading prompts can be fun too. I guess what I was aiming at was some kind of 'measure' of the activity level.

The activity level matters because people won't get motivated to participate unless they feel like other people are participating too (which is a kind of prisoners dilemma). I'm not quite sure how to solve this.