As a recent daily user, prismatic is awesome. The way they leverage twitter is pretty interesting, mixing in "endorsements" to articles from both people close to you, famous people, and random people, in a way that keeps it interesting. There's also fairly heavy machine learning going on behind the scenes, in a way that is non intrusive. Congratulations to those guys.
Agreed, I think it's the first effort at article recommendations that's really worked for me. I think it's due to a pretty clever combination of social signal and machine learning.