Well Check 6, here is how it works from a passenger's point of view - based on may year's experience and over 2 million miles of being SLF. Complaints through the airline's system sometimes get a response, sometimes don't (more likely if you are FF gold). When responses come they can take months and can be dismissive.
BUT, if I end my letter with "You have three weeks to respond to this complaint, after which I shall tell the story on the following web sites......." WOW!! - personal emails, upgrade certificates, miles added to my FF account pour over me in no time like the gentle rain.
Large airlines are just like any other cost constrained bureaucracy - they are driven by procedures, not service. Once you figure that out you have the perfect formula for getting the best possible service - ALWAYS be pleasant and polite to the front line staff on the spot and ALWAYS be an absolute stand out bastard to the managers you never meet face to face in ways that are targeted on their performance criteria.