The other problem with most online communities is that there's almost no limit on output. All else being equal, someone with an active offline life is going to

The other problem with most online communities is that there's almost no limit on output. All else being equal, someone with an active offline life is going to have a smaller presence than someone who's terminally online, someone who spends more time listening to others is going to have a smaller presence than someone who spends more time talking, someone who puts more effort into their posts (proofreading, look up evidence) is going to have a smaller presence than someone who haphazardly shoots off comments, and someone who reserves comments for when they have something important to contribute is going to have less of a presence than someone who comments on things they don't know anything about. Things like mods (most people don't have the time commitment), voting (terminally online people will be able to vote earlier and more often, driving the conversation), front page thresholds (a small group of people acting as filters for new submissions), and time limited submissions (the longer you wait to comment the more likely the conversation will have died) just skews things even more. For all the discussion about the low quality of online conversations, there seems to be very little discussion about the environments we've created and what kind of discussion these environments encourage.