how to get my app back

Solution
It may have been deleted from the home screen but that doesn't mean it is gone from the phone. First try restarting (turning off and back on) the phone. That may resolve the issue. If it still is gone, then tap and hold anywhere on the background of the home screen. This should bring up a screen that gives you access to icons, widgets, etc. Locate the facebook icon in there and then readd it to the home screen.

If by some chance the icon isn't actually in there, then the app may have been removed from the phone. Go into "Settings" and then "Apps". Go to "All" and search for it. If not there, then you just need to reinstall it. If it is, however, there, then it sounds like it is having problems and you should then uninstall it, restart...
It may have been deleted from the home screen but that doesn't mean it is gone from the phone. First try restarting (turning off and back on) the phone. That may resolve the issue. If it still is gone, then tap and hold anywhere on the background of the home screen. This should bring up a screen that gives you access to icons, widgets, etc. Locate the facebook icon in there and then readd it to the home screen.

If by some chance the icon isn't actually in there, then the app may have been removed from the phone. Go into "Settings" and then "Apps". Go to "All" and search for it. If not there, then you just need to reinstall it. If it is, however, there, then it sounds like it is having problems and you should then uninstall it, restart the phone (turn it off and back on) then reinstall the app.
 
Solution