NPS is a type of survey that is widely used to measure customer loyalty. But it has been highly criticized. NPS often fails to predict loyalty and having to choose between 11 different scores is a hard choice to make for customers resulting in some inaccuracy.
In addition to the traditional question "How likely are you to recommend the product to a friend?" you absolutely need a qualitative question to go beyond a simple number and better understand your customers. It takes the form of the following open question "What is the most important reason for your score?".
Going deeper than the three NPS segments – detractors, passives, promoters – will also help you better understand customer behavior. There are many metrics you can use but it's up to you to see what works best: pricing plan, customer feedback, user activity, etc.
Although promoters are likely to refer you to a friend, they probably won't. Follow-up with your promoters and actively seek their help:
NPS isn't accurate enough to use it by itself so you will have to complement it with other metrics. But the real benefits also require a lot of work. Following-up takes time but the resulting growth is going to be worth it.