Here’s how you can see which friends you’ve lost:
1. Enable the new Facebook timeline.
2. Pick a year in the timeline and locate the “Friends” box.
3. Click on “Made X New Friends.”
4. Scroll through the list. Where you see an “Add Friend” box, you know that you are no longer friends with that person. If you haven’t unfriended them, they’ve unfriended you.
There’s still no real-time Facebook alert for an unfriend. But if you’re really interested in learning more and more quickly about your declining online popularity, a browser plugin like Better Facebook is a good alternative which has helped many companies and businesses clean up their profiles.