ReadBurner doesn't focus on aggregating shared items from your friends and it's not restricted to Facebook: the application gets data from any shared items page. "In order to filter out the best stuff ReadBurner counts whenever an item is shared by multiple persons. Since Google Reader users can generally be described as very tech-aware, I think that over time lots of good stuff can come out of ReadBurner," explains the author of this project, Alexander Marktl. For now, most of the popular items are related to technology and a post can get at the top of the list if 20-30 people share it.
Louis Gray, who found this site in the referrer logs, thinks "there is a market for people looking for most popular shared items, and finding new people sharing what they read on Google Reader." It's not clear why Google Reader doesn't experiment with this idea, at least by de-duplicating the reading list and showing only once the posts shared by more people.