FollowMeter has been around long enough to build a real user base and a reasonably polished iOS and Android app. It shows follower counts, non-followers, recent unfollows, and engagement approximations. For a lot of users it does what they need. The problem is the access model. To pull any of that data, FollowMeter authenticates with your Instagram account using your username and password. Your credentials leave your device. Where they go after that, and how they are stored, is not something you can verify.
This matters more than most people realize. Instagram actively detects and suppresses accounts that connect to unauthorized third-party services. The platform uses behavioral signals and session fingerprinting to identify non-human access patterns. An app logging in as you, on a server, from a data center IP, is a recognizable pattern. Some users lose access to their accounts. Others face repeated checkpoint challenges.
WhoUnfollowed never touches Instagram's servers. You download your own data directly from Instagram's settings panel, a standard export that Instagram is legally required to provide under GDPR. You upload that ZIP to WhoUnfollowed, and the app parses it locally in your browser. Your followers list, your following list, and the difference between them. If you upload a second ZIP from a later date, you see exactly who unfollowed you in between. The file never reaches a server. The app never touches Instagram.
FollowMeter gives you convenience. WhoUnfollowed gives you the same information without the exposure.