Mark Zuckerberg won’t tell you his favorite Facebook application (Charlene Li just asked him on stage at the Web 2.0 Summit) but I will. It’s Feedheads (just was renamed from Google Reader Shared Items App). Louis Gray has the details.
If every app were as well done as this one Kara Swisher wouldn’t be able to call Facebook apps “toys.”
You all should spend some time understanding what this app does. It’s deep and uses the social graph in a way that I haven’t seen any other app do.