Hi, I'm Ashik. Welcome to JustAnswer. I'm reviewing your question now, and will post back with your reply ASAP
This happens if there is any issue with the network connection or iTunes account. Try the following the solutions;
First we can reset your iPad and see if it works or not. Press and hold down the Power button and Home button at the same time until it shows the Apple logo. When you see the Apple logo, release the buttons and let it boot. Then go to Settings > iTunes & App Store. Tap your Apple ID and tap Sign Out. This will sign out from your account.
Then open App Store. Tap Updates tab. If you see Update button next to the Facebook, then do the update. Sign in if asked.
If it is still stuck at installing, then you need to delete Facebook and redownload them from App Store. In home screen, tap and hold down Facebook and it will start wiggling. Then tap the x button on the app to delete it . Then open App Store and redownload Facebook.
Please let me know if that didn't work, so we can continue troubleshooting.