But the employer was monitoring the employee's activities on company-owned infrastructure... (I think the messaging is a different story than the location data)
But we are talking about employees communicating on the corporate platform.
This gets a lot clearer cut in my opinion if instead of Facebook, we say EvilCorp, and instead of "chat messages" we say we're talking about "company emails."
If I send my coworker at EvilCorp a number of emails on their work account on Tuesday about how we're totally going to get drunk on Wednesday instead of going to work, and they respond in agreement, do you still have a problem with the employer digging through those comms when the employees do a No-Call No-Show on Wednesday?
I mean this seriously; if you're going to coordinate your planned truancy-from-work event with other co-workers and use work email to do it, you should expect to get caught. You might find it scummy, but I don't think there's a court in the world that is going to declare that it was illegal, except maybe in Germany or some other country with super-privacy laws. Educate me for sure, if you know I'm wrong.
And even then, I suspect that there's a language they could include in their Employee Handbook that would make it OK. The company network is for company business. You're not usually entitled to hide your work-product from your boss. More seriously, something in the art of OpSec is lost today. If you work at Facebook and do this, at least use Hangouts.
Then I elaborated and said the same thing, but more detail. This comment got one downvote and no replies.
Explain yourself please, whoever downvoted. I want to believe that you aren't just downvoting because you disagree.