I don't think this would work very well on Reddit unless you applied some sort of correction factor to account for the size of the subreddits that the user post

I don't think this would work very well on Reddit unless you applied some sort of correction factor to account for the size of the subreddits that the user posts to. For instance, someone who posts regularly to /r/luthier would have far fewer unique users upvoting their posts than, say, /r/pics, which is a much larger community.