I worked in admin for a private company for five years, was a horrendous place to work. Such a toxic environment where I was bullied by my supervisor and manager on a daily basis.
One weekend I was doing filing and my manager, the silly woman, had left printed out personal messages from my personal Facebook in the filing cabinet, obviously forgetting I was in over the weekend.
My heart sunk. These were messages of me completely and utterly slagging both my supervisor and manager off, saying really mean and awful things, slagging off how they look, complaining of managers bad breath, however they were sent to my best friend over private messages via Facebook messenger so irregardless of what I said, they were in the wrong.
It turns out, that I had left my Facebook logged on (I was allowed to use personal Facebook to get the company short notice cover) and my supervisor had noticed this and went ahead and read all my messages. She then changed my password on Facebook so I was locked out for days and had to send them a picture of my passport to allow access to my account.
I raised a formal grievance about this of course, I brought along my union rep. My manager and supervisor would not take any responsibility and still put the blame on me saying that I shouldn't have said those things and then completely lied and said I was never allowed on personal Facebook for company cover and so I'd breached internet use policies for being on Facebook at work. This was completely false.
There was no internal HR, HR was outsourced to keep management right and after the director of the company 'investigated it' she was best friends with the manager - it was never upheld.
They then tried to move me to the kitchen to work after five years in the office. Suffice to say I sent them all an email telling them to fuck off and exactly what I thought of them all. Don't regret it to this day.
I'd say you don't have a leg to stand on.