multiple users per device are already supported if the users have different user accounts on the OS. On Android, you could have one account for your kids for example. For the same user account, you cannot have multiple users though, as this is also not supported by iOS Game Center or Google Play Game Services afaik.
currently there is no support for multiple users on the same device (AND on the same user account). A device can only be connected with one user, as it is the case for Game Center & Google Play. You could disconnect the device though and then continue with connecting to another facebook user.