I'm not sure if it's sane, exactly, but this post got the ol' gears spinning, and I started to think in terms of an activity score that would go up with posting, commenting, etc., and down with days since last contact. It would need a table of its own, just to associate userids with scores and last update time of the score, updated whenever the user "does something".

Then I thought, hmm, I wonder how Sourceforge does project activity rankings... They're comparing several 7-day stats for each project to the highest 7-day activity of all time, over all projects. Definitely some food for thought there.