What's the most addictive non-game app you've used, and why does it work?

I've been deep in gamification research while building a study app, and it's made me pay way more attention to which non-game apps actually hook me. Duolingo's streaks, Snapchat's snap score, even a habit tracker's little dopamine hit when you check something off, they're all borrowing the same mechanics games have used forever.

Curious what does it for you. Is it a streak you don't want to break, a collection you want to complete, a leaderboard you're quietly competing on? Or something else entirely?

Trying to figure out which of these actually hold up outside of games versus which just feel gimmicky once the novelty wears off.