It may not have vanished. First, go into "Settings" and then "Apps". Scroll over to the "All" list and scroll down to locate the Facebook app listing. Is the Facebook one still listed there? If so, the app is still on the device. Don't do anything in here. Exit the settings and go back to the home screen.
NOTE: If it isn't listed in settings and apps, then it was uninstalled from the device and would have to be reinstalled.
Now you just need to find where the app's icon went.
First check all your home screens and open any folder icons. It is completely possible the icon got moved into a folder with other apps. Then all you have to do is tap, hold and drag it back to the home screen for it to be back there.
If, however, you do not find it in any folder or combined in a folder with any other app, then you will have to add the icon back to the screen. To do that just tap and hold anywhere on the empty background of the home screen. Then look through the available app icons and find the Facebook one. Add it back to your screen and you should be fine.