no one is manually checking thousands of celebrity accounts. it's all scripts/bots checking accounts for changes in the "following" with probably a rule for prioritizing notifications based on the unfollowed follower count (e.g alert me if this unfollower has 200,000+ followers)
edit:
sort of like this old script
A tool to scrape the list of followers and people following a user and see the difference with the last recorded list - tonoli/instagram-followers-scraper
github.com
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A tool to scrape the list of followers and people following a user and see the difference with the la