There are analytics services that help track who follows, who unfollows, and how your audience behaves, but they work with authorized data and don’t violate privacy. You can read about one such resource and discussion at https://intua.net/forums/index.php?p=/discussion/13496/an-app-that-shows-who-follows-and-unfollows-whom-on-instagram which shows who exactly leaves and who stays among your followers.
