Missing Facebook notifications on iPhone

1. Help for Notification Alerts --> No Notification Alerts - Apple Community
2. To turn ON notifications selectively for apps, go to Settings > Notifications > Scroll Down to the App and turn ON notifications... Change notification settings on iPhone - Apple Support (IN)
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3. Some apps have their own notification settings over and above the iOS notification settings: An example of Instagram Notification settings is given below
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I’d contact Facebook support and inquire about the issue. I believe there’s a phone for their support and you might be able to contact them via X (formerly Twitter).