From the training I’ve had, then you appear to be on very dodgy ground. You don’t need to name the company, if any association you have will be apparent.
Deleting after the event isn’t going to help if they want to make an example of you.
They don’t need a specific social media policy, there will no doubt be a clause in your contract about bringing the company into disrepute. Social media policies are becoming more common, but it just backs up what’s already there.
Hopefully they will just rap your knuckles, but be prepared. In a union? Call them now. For any disciplinary proceedings, always take someone in with you.
When I was briefed on this sort of thing there were plenty of cases of dismissal used as examples.
Basically it boiled down to ‘don’t bad mouth your employer on any social media’.
By the way, this is also social media, so you need to think very carefully about what you say on here. Repeating what you said on Facebook on here is pretty much the same as putting it on Facebook in the first place
Good luck!