What's the fraction of lurkers vs active users?
unalone: I think Hacker News might have a significantly lower rate of lurkers, because of the fact that it keeps itself out of the public eye. OmegasEye, an old site of mine, had very little lurking, because the people who knew about it were usually its members. Hacker News isn't that obscure, but it isn't a site that attempt to make itself high-profile.