If you aren't exceeding your customers' expectations, you can bet they're looking for better alternatives. You can either make it hard to shift to those alternatives, or match and exceed their expectations. Understand what those expectations are and use surveys to discover your customers' pain points. Help them overcome troubles they never knew they had and invest in retention.
All these might sound like an increase in your costs, but it costs a lot less to retain a customer than to acquire one. The longer they stay with you, the more loyal they tend to become to your products, services, and brand. Customer loyalty is the cornerstone of repeat business. Loyal customers will always choose your brand over a competitor. The only way to build loyalty is to do right by them, and that means listening to them and meeting their needs.
Churn rate is the rate at which your customers are leaving. If you had 100 customers at the start of a month and 15 of them cancel their subscription by the end, then your churn rate is 15% for the month.
Retention rate is the complement of your churn rate. In the above example, your retention rate is 85% (100%-15%). Higher retention rates imply higher loyalty among customers.
However, a short term rise in retention can be achieved by simply making a reactionary move to meet customer expectations when you're bleeding customers. A more reliable way of measuring long term customer loyalty is by conducting surveys periodically over a long period of time.
A Net Promoter Score (NPS) survey can measure your customers' satisfaction towards you, and indirectly measure loyalty. Your customers' answers to questions like, "How likely are you to recommend our company/product/service to a friend or colleague?" can be scored to indicate overall customer satisfaction. Higher ratings mean more loyalty.