Number of complaints/compliments
If the company get plenty of complaints this implies that they're delivering bad customer service. They will be able to see if the complaints they are receiving are similar or different. If they are similar, it will be a sign that they are clearly doing something wrong and needs to be corrected immediately if possible.
Compliments are the same except they are doing the right thing and should continue to keep doing it. example, virgin airways got a complain from a customer which was six pages long.