Haha, the "fun factor". I think I'd rather rely on something concrete like number downloads rather than this nebulous concept that probably nobody can give a definition everybody can agree with.
Fun factor isnt a good way of describing it. Its basically an algorithm that looks at total time played on that game, and how long people play a given game per session. The games with the most total play time, and time per session are probably more fun than games that people play for 2 minutes. I believe they have plans for sorting by day, week, month, and lifetime. I forget my source though