To determine if Facebook advertising is a waste of money you have to know the objective. If you simply put an ad up and you have no idea why you are doing it or what you want out of it then you more than likely are wasting money.
To have a clearly defined objective that is directed to a very specific audience who is likely to want to purchase what you are advertising then you are not wasting money. So it is up to you or your agency to make sure your ROI is what you are looking for. If it is then Facebook ads are not a waste of money.