I go by posts more than I do time . . . I could log in and leave the forums minimized on my computer all day if I wanted to.
See, I do that in a sense. I bring up the forums, and then I check them about once every half an hour or so.. and reply, or just read if I want to. However, number of posts is a good indication how how much a person participates. Then again, you could do something like posts per minute, or something that might be a good indication of how active a person is. That would be neat.