You are suspended while an INVESTIGATION is carried out. If the investigation leads to a disciplinary, you should be given full details of the allegations against you, and you will then get to see what the company are complaining about.
You are clearly guilty of bringing your company into disrepute (sorry!) as one of your FB 'friends' has forwarded what you describe as a 'rant' to your boss. Obviously the company will react to this - and the post was obviously more 'public' than you realised if someone has complained about it. I would not think it necessary to have a published policy which specifically stops employees from bringing the company into disrepute, and in any case you have no employment rights, as you know.
In order to cancel your leave, the employer needs to give you notice of at least as long as the booked leave, so if you are off next Wed for a week, your employer cannot cancel this. Yes, go out and carry on life as normal (obviously avoiding contact with prohibited people as far as possible.)
And yes, job hunting is in order, I'm afraid...... If you are dismissed, you can be upfront with future employers, saying that what you thought was a private conversation turned out not to be.
(WHEN will people learn not to talk about work on social media?

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