Customer surveys are worth the paper they are printed on (or the electrons used for an online survey). Unless your survey is well designed, you will always suffer from bias. It is this bias that makes interpretation hard. Sending a form to all customers, does not mean you will get back a representative sample of the customers.
Having said that, measuring customer satisfaction is usually accomplished through analysis of a complaints data base (another faulty measure). Monitoring complaints or percent complaints or complaints per 1000 units, etc are just surrogates for customer satisfaction. The real issue is can you take action and improve on the data you received even though it might be flawed.
I worked with a customer who measures complaints per month. They have an internal requirement that complaints must stay below 100 per month. They average about 10 complaints a month. I got involved in helping to reset the requirement because they got an audit finding when they had a month with 75 complaints, but did not take action. We had to create a statistical method to "normalize" the complaints because the 75 complaints also reflected a 35% increase in sales, therefore leading to a higher customer base to complain. When normalized, the 75 complaints was no different than the 10 complaint average they were used to seeing.
I used that example to give you an understanding that measuring customer satisfaction is not easy, and having a metric or improvement plan for it is even harder!