Putting an employment ad in a magazine costs lots of money so regardless of the moral or legal implications many operators (especially the smaller ones with less money) will use many different methods to recruit, including but not limited to:
Recruiting qualified pilots who already work for the operator in another capacity ie cabin crew, engineering, ops etc.
Asking pilots to recommend people that they know who are looking for work.
Employing family members of senior employees.
Poaching from other similar operators.
Approaching flying schools for recommendations.
Contacting someone who happens to submit their CV or visit the office on the day that a pilot hands in their notice.
All of the above are valid ways to get into an operator and none of them require a formal recruitment ad.