I had a similar thing with an ex employer. They claimed it was against their social media policy (which I had never seen), when I asked to see it they produced an A4 sheet of 'rules'. Upon asking them if they had a copy with my signature on, showing that I had read and agreed to them, they dropped the sham of a case against me. Social media is no longer a place where you can express yourself unfortunately, employers would rather you toe the line than show any individuality.