To download Facebook for iOS 5.1.1 on an iPad (such as the 1st generation iPad), you cannot simply search for it in the App Store, as the current version is not compatible with such an old operating system.
Tap the button. A pop-up will appear stating: "The current version requires iOS [X.X] or later, but you can download the last compatible version" . Tap Download to proceed. Alternative: Using the Web Version
: Tap the cloud icon or "Install" button next to Facebook.
Apple has a hidden feature in the App Store for legacy devices. If you have previously downloaded Facebook on this iPad (or any other Apple ID account), you can download the last version of Facebook that was built for iOS 5.1.1.
