I don't know if you can say Digg is after the "Geek Culture" demographic anymore (as per the chart in the post). It seems to me to be a centralized replacement for the kind of people who would constantly forward emails to everyone in their contact list. Ok- it still has a 30% geek content.
I wonder how much more social news can evolve without any new advances in the underlying technology. Without new tech (digital identity, semantic web, etc.) then the only thing you can do is to change the people present or the way that they interact. And if there is significantly more value that can be extracted by tweaking the design then it'll take someone smarter than me to see it.