Say you let me have a copy of your house key (in case you lock yourself out or sometimes I come in to water your plants maybe watch a little TV and you are cool with both of those). Someone then asks me for the key, and I let them have a copy of it. Do they have your permission to enter your house at all? I sorta think not.
Permission to enter with a key does not necessarily transfer with possession of the key.
How do you get an analogy between a facebook password (which you pick by the way not them) and "copy of your house key"?
The analogy would be you rent a house and are given a key. And then you can give that key to someone else (like the cleaning person). You can also give the key to someone on Airbnb but that might be prohibited by other language ("you can't rent").
Your key example would be "does the employer have the right to give your password (which you gave them) to someone else". The answer to that is obviously "no" unless you told them it is ok to do.
Your Facebook user name and password is exactly like your copy of a key that lets you into their house. Your having picked the character strings used doesn't matter. You are entering their system when you use that key.
Them letting you put a bit of your furniture in their house (i.e. content you own) doesn't mean the house itself is yours. Nor does it mean that your permission to enter their system necessarily transfers to other people even given that you happen to give those other people a copy of the key.
It's probably not productive to use poor analogies here.
Accessing a computer system without authorization from the owner of that system is a crime. Facebook doesn't give authorization for employers to access their employee's accounts on their system, and explicitly forbids their users from transferring their own authorization to others.
As far as I can see, there is simply no way to construe an employer's access as authorized by the owner of the system i.e. Facebook.
Permission to enter with a key does not necessarily transfer with possession of the key.