If karma in a social site is a reward for good behavior, it seems like it always becomes harder and harder for new users to "get ahead" compared to those who have been around forever. Perhaps this lessens the incentive to good behavior over time?
Do such sites need to implement some kind of "karma decay" system, to prevent the leader-board from becoming more static over time and encourage new users to behave themselves?
I have been here since fairly close to the beginning (for the public anyway), and have moved around in the rankings quite a bit. Since I quit my job in May, I have sunk from 64 to 100. (I don't spend several hours every morning reading HN with my coffee, since I'm rarely up in the mornings now. ;) I can assure you that newer people are able to push ahead of older people if the older people stop participating.
For instance, I have been on here five times as long as qhoxie, but he has three times the karma I do. The reason the leaderboard might look stagnant is that the people in the top 10 are there for a reason... they are consistently more productive (in the karma sense) over the long run. If they started resting on their laurels, people like qhoxie would rapidly overtake them.
Also, as the site grows, it gets much easier to rack up tons of karma. At one time a story was really hot if it had 20 votes. Nowdays it's common to see things go well over 100. I was once really surprised to get 26 karma for a comment I made. Nowdays there are gems on here that regularly go over 50. More people voting means the new users actually can catch up faster.
(Of course, this also assumes that karma actually matters, which I debate. It's nice to know people appreciate what you said/posted, but you shouldn't just be saying/posting it to win a karma video game.)