If you ever have to uninstall the app again here is something that may help.
1. On your device, open the App Store app.
2. Click the button at the bottom to navigate to "Purchased."
3. At the top of the purchased apps page, click the button that says "Not on this iPad/iPhone."
4. Scroll through the apps until you find the Diet and Fitness tracker and click to install.
as for not being able to log in using facebook, do you need to reset your password?
"We either make ourselves happy or miserable. The amount of work is the same." Carlos Castaneda
Live Kind. Laugh Often. Love Always.