I think it's reproducible if someone building a web community wants to have a similar level of user community - but it requres doing the two things that typical web startups won't do - have paid, full-time moderators who are in the top tier of intelligent users, and don't just try and grow the site forever.

Reddit is a good example, the subreddits are often as good as any other web community, but the site as a whole is trending down to Digg-levels and worse.