Tap the (the square with an arrow pointing up). Select Add to Home Screen .
Tap on your profile icon in the top right corner, then select Purchased . Download Facebook Ios 12-5-7
If you want, I can provide exact menu navigation for your specific iPhone model or walk through the Purchased workaround step-by-step. Tap the (the square with an arrow pointing up)
Since you cannot download the app directly from the App Store front page, you must use one of these "legacy" methods: Download Facebook Ios 12-5-7