I say work backwards. Instead of thinking how you want to compute the score, think about how you want the distribution of scores to be. Anything you come up w

I say work backwards. Instead of thinking how you want to compute the score, think about how you want the distribution of scores to be. Anything you come up with will be fairly arbitrary, so you might as well "game the system" by creating incentives that will cause people to get addicted to increasing their score on your service. For example, you may want very few people in the 90-100 range (reward the few who reach this high level), very few people in the 0-10 range (don't make people feel bad), and most people below 50. Then come up with a scoring system that fits this distribution. If you find a metric that doesn't quite fit the distribution, tweak it by using a log transform, or by adding to/multiplying everyone's score. Also, make it simple to understand. If it's some magic formula that people can't understand, they may not engage as much. Make it clear what they need to do to increase their score.