-When you click on "Check now" button app request your facebook friend list. After your approvment app get your friends and store it.
After a few days later when you come back to check, then app again get your current friend list and compare it with old one.
if someone unfriend you or you unfriend someone or someone change his name, surname, blocked accounts, deactivated accounts and etc.
any changes in your friendlist app detect it and show to you.
Who unfriended you
Whom you have unfriended
Who blocked you
Who deactivated his account
Who changed his name