TweetEffect is a quick handy tool to find out which of your Twitter updates made people follow or leave you. Simply enter your twitter name and TweetEffect will analyze your 200 latest updates and flag those up that made people follow or leave you.
Obviously there might be other reasons why people come or go, nevertheless it is an interesting indicator about the effect your tweet might have had on the world. It is free and easy to use – no login, no registration required. Try TweetEffect!