How does the app, "Friend Watcher" Work?

This app monitors your list of Facebook friends and lets you know when they change their name, add you as a friend, deactivate their account, restore their account, delete you, block you or unblock you etc. Now regarding hacking, I don’t know how far it’s possible. But you can just be careful by taking some precautionary measures like hiding your Email Address from everyone (Edit Profile>Contact Information>Clicking on the icon beside your email address> checking ‘Only Me’). You can activate ‘Secure browsing’ in Accounts setting and check ‘Send me an email when a new computer or mobile device logs into this account’ and click Save. Also avoid clicking on suspicious links.