Web: Do you want to know which link is being shared most often at the moment on Facebook, or retweeted by most users on Twitter? Rad URLs is the place to go to. The webapp has a simple list of the 100 trendiest links at the moment across the two social networks.
You can sort the list by links being shared most often on Facebook or Twitter separately, or use the combined "Trending" algorithm — although there are no details on how this works. Each link has a headline as well as the source website, and hovering over the link will give you a preview pane.
I've been using Rad URLs for the past few days and it actually works pretty well. It's updated every 15 minutes, but a major change in shared links happens more around the hourly mark. Still, it's a cool place to find out the most popular stuff on the net right now.