If nothing else, it's legitimate to filter non-participants before doing a regression analysis on the rest. It's technically true that you can't predict karma over time, but that's not really an interesting statement until you eliminate the large number of people who sign up, then never post or comment.

My instinct is that once you filter these people, you'll see a much stronger linear relationship between time and karma, since karma isn't normalized by the number of contributions, and number of contributions is probably a poisson process.