There could be ribbons and icons, like for the post count. Or maybe just specific member titles. Like "Obsessive lurker."
And like all "experience points," having more is a clear indication of a lack of life
Another "measurement" could be combining the join date with the post count. Each on their own doesn't really mean much, but deriving an indicator from the both of them could be fun. Let's say a year is worth a thousand posts (or five hundred). Or the other way around. The years + posts could represent a "level."
For example, [___]drian has been here since 2004., and has 11 thousand posts. That would put him at level 16 (Thordrian, Lv16 Mod... hee hee, sounds good). I've been here for three years, have two thousand posts, so that would make me level 5 (almost 6, yo
Of course, inactive members would still level up, but you don't see them around.
Think about it.