Although you shouldn't live and die by how many Twitter followers you have, the number could be important for certain people. In March 2011, the New York Post noticed that the popularity of various "American Idol" contestants had a correlation with how many people were following each singer's tweets; contestants with fewer followers got voted off, those with the most followers stayed on the show, and so on. Even if you're not competing for a record contract, you can easily see how your follower count stacks up against your friends.
Visit the user's page on Twitter (twitter.com/username).
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Look for the "Followers" listing on the top right side of the page, under "About @username." Next to the word "Followers" is a number; that's the person's number of followers.
Click on the "Followers" link to see a list of who's following that user. This list will only be visible to you if the user's tweets are public or if you're following the user; protected accounts' follower lists are hidden from view, although general follower counts are not.