If you are looking for a twitter tool that alerts you whenever anyone stops following you, then try TwUnfollow. TwUnfollow is a free service that monitors your followers and notifies you from time to time about people who are not following you any longer. The monitoring schedule is 4-5 times a day.
The time duration for notification is configurable. You can decide to get an email instantly whenever someone unfollows you or as a daily digest of unfollowers. You can also see the history of recent followers and unfollowers.