Twitter does have several options to know when users unfollow you, however, what about Facebook? How do you figure out when someone unfriends you on Facebook? Turns out there is a way to do that too.
Unfriend Finder for Facebook is a Greasemonkey script which tracks your Facebook friends and then notifies you whenever someone decides to unfriend you on Facebook.
The script tightly integrates within Facebook and sends you notifications whenever someone removes you from your list. It also alerts you when someone ignores your friend request.
The only problem of the script is that it is browser specific and only works on Firefox, Google Chrome, Safari and Opera right now. There is no support for Internet Explorer yet, but you can read up some tips on running Greasemonkey scripts in IE.
You can download the script or add-on for your browser from here.