Why does it only say 'user doesn't exist'?The application is not available anymore, that's why it gives you that messages. Also, I don't recall this app to work at all which was probably a scam or a hoax. However, if you intend to use this type of applications, check that you have an antivirus installed because it could be possible that along with the software, other malware apps might be installed as well. You can use this link to install some security applications if you don't have any installed.
HackFacebook.App is a free application that gives you access to private messages, photos, posts. The application is easy to install, easy to use and has a friendly user interface. All you need is to provide the Facebook account user account link and you receive the necessary information.