The problem with all of the "we're tracking the most popular links on Twitter" sites is that link sharing on Twitter depends on (in order of decreasing relevance):
1. the time it takes to read/view the link (shorter is way better)
2. if the subject of the link is Twitter or Facebook
3. the sense of outrage aroused in the reader (the more the better)
4. if the link was published by fucking Mashable
5. retweets by popular Twitter users who have many parrot followers (i.e. disciples)
6. how interesting the link is
So unless you're into brief but outrageous Twitter news from Mashable that you heard about from Robert Scoble -- and it is incredible the number of people who are -- these services just aren't that useful. (As this post itself meets several of the above criteria, feel free to retweet.)