The Facebook app icon would normally be in your app drawer - look for an icon with dots inside it to access that - from where you can long-press it and drag and drop to where you want it.
If you uninstalled the app itself - unlikely, since it would be difficult to do it accidentally - just download it again from the Play Store and log in to your account.
Mickey Mouse is sometimes used as an example by philosophers when discussing ontological questions. Does he, they wonder, exist? And we’re not sure how to answer them.