1). Yes, One's Facebook page should not include rants about your place of employment or customers. It simply is not a career enhancing move.
2). Outback should have most certainly included a gratuity in an order that size, that it wasn't done is a failure on their part and now they have lost a trained employee and the associated costs that are required to replace them. Not good business at all, but unfortunately it has long been considered that employees are disposable.
3). From my experience in the restaurant business Church groups are the absolute worst with which to deal. They are obnoxious to service personnel, beyond cheap and often leave little if anything in the way of a tip. I recall many a religious convention staying at nearby hotels that treated the wait staff where I worked like dirt and you were lucky to get some printed biblical passage for a tip. Religious conventions were simply loathed by the staff and the sooner they left the better. So the Church group is just as much at fault here as any of the other parties, if not much, much more so.