One flaw with Twitter: it tells you when people follow you, but not when they unfollow you.
Unfollows can be a useful gauge of your twitter performance, particularly if you get a rush of them after a particular tweet.
There are various services that analyse your Twitter account to identify those you follow but who don’t/no longer follow you.
Here’s a top list, as recommended by the members of the Twitter Strategies group on Linked In.
Your Twitter Karma http://dossy.org/twitter/karma/
Brings up a clearn interface with your follows and their pix, with a green 'you follow' flash and a red 'they follow you' one, so non-reciprocals are clear. As are those who follow you but you don’t follow back. There’s a button to unfolow/follow on each entry.
Manage Flitter http://manageflitter.com
Wasn’t working when I tried it, but here’s a quote from TechCrunch they use on their home page: "ManageFlitter is easily the best... It's a must-use." - TechCrunch
UseQwitter sends you a summary email at least once a day informing you of those who have stopped following you on Twitter.
They say: “SocialToo helps you manage Twitter and Facebook and unclutters your stream and Social Graph so you can focus on what's most important - relationships!”