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Messenger doesn't delete your old messages on its own, they stay there unless you delete them or you delete your account 😊 But if you're looking for something old, the search function can be a bit finicky, so try scrolling through the conversation instead - it's often more reliable than typing in the search bar.
Don't think Facebook's gonna automatically archive them or send them somewhere else 🙂 I've got conversations from 2015 that are still there, you just gotta scroll for a while to find them. The thing is, if you switch phones or log out, it can take a bit for everything to load, so it feels like they're gone, but nah they're just still syncing.
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