The Unfriend Finder was an app that could be used with Facebook that allowed users to determine if other people had defriended them. Users found Unfriend Finder useful because its capabilities were not part of the native Facebook platform.
When Unfriend Finder was available, it integrated as an extension to Web browsers such as Firefox, Google Chrome, Opera and Safari to notify users whenever a Facebook friend defriended the user or when a friend deactivated his profile. Unfriend Finder accomplished this by injecting its own code into Facebook's source code before displaying it to the user.
Facebook determined that this was a violation of its terms of service, and as of 2015, requested that Unfriend Finder not be used. According to the Unfriend Finder's official website, the creators of the app complied with Facebook's request.