Q. How can I see who quit following me?
A. There are a few services that let you know this information. One that we like is http://useqwitter.com/. You do not need to enter your password, just your username and e-mail address.
This service tells you when someone unfollows you, who that person is, and your last tweet you posted. Keep in mind that your last tweet is not always the reason you were unfollowed. It could be one of literally hundreds of reasons.
Also, do not take unfollowing personally. It could have absolutely nothing to do with you or your account.