Here's an interesting post by zgiltner from a Human-Computer Interaction blog on the Sociability of reddit.
My favorite part of the analysis is below -- the observer rightly addresses the striking differences between communities.
There are many different subreddits which coordinate the where people may look for certain subjects. In the screenshot I posted the article was submitted in the politics subreddit. The entire culture of commentors varies vastly from the “bacon” subreddit to the “vegan” subreddit. This allows users to narrow down their conversations. Users have been known to post other articles related to the discussion of the conversation, allowing further research on whatever topic and continuing conversation can happen.