Digg is more like Facebook and Twitter than StackOverflow. Each of Digg's logged in users gets their own "News Feed" based on the users they follow/friended/are

Digg is more like Facebook and Twitter than StackOverflow. Each of Digg's logged in users gets their own "News Feed" based on the users they follow/friended/are most similar to. StackOverflow on the other hand is much simpler - questions and responses, plus users and voting. AFAIK there's no collaborative filtering going on at SO, be it user or item based. I don't know why Digg needs so many boxen, but I did find Spolsky's comparison disingenuous.