Android tablet separate user account - app installed once or multiple times?
The second user will have to explicitly install the wanted app (in your example: Facebook) – as otherwise all accounts' app drawers would be cluttered with a bunch of apps not wanted by everybody, except for those who installed them. But if the app already had been installed before by another account, it will rather be a "fake install" (i.e. it appears as if the app would be installed anew, but it's not downloaded again from Playstore, and thus doesn't consume "double space").

One side-effect to that is: whenever a user decides to update the app, that update applies to all other accounts of the device as well.