Sorry about the confusion: by post I meant either news stories or comments, no distinction. We could use the median rather than the mean, but I see no problem

Sorry about the confusion: by post I meant either news stories or comments, no distinction. We could use the median rather than the mean, but I see no problem in the fact that a few very popular comments can pump up your average. They boost your absolute count too. And that's OK. Karma measures popularity.