Jason Calacanis has some worthwhile stats up (go 2/3s of the way down) on how reader contributions are key to some Weblogs, Inc., sites:
AdJab is getting half a comment to every post.
Cinematical is getting 2.3 comments to every post.
TVSquad is getting 5.2 comments to every post.
Clearly TV has the water cooler effect going (thanks to 100+ comment posts about American Idol winners/losers each week).
What do I take from these stats? Well, each comment is about half the size as the origional post on average. This means 1/6th of AdJab’s total content is from users, Cinematical is 33+ user content, and TVSquad is 40%+ user content.