By asking this question, the interviewer wants to evaluate your understanding of different metrics and wants to look at what metrics you think are important to measure the success of a business.  Generally, a campaign of a business intends to achieve four results in the following sequences - Increased engagement, Increased reach, increased leads, and Increased customers. When you go on to answer the question, it is better to stick with these four elements only and set your KPI accordingly. When it comes to increasing engagement, you first need to see how the business engagement has been currently, you can use CTR as your metric here to measure that, and similarly, you can use different metrics to assess the brand’s performance over different aspects of the result.   If you find the Clickthrough rate dropping, you can act upon it and try to change a thing or two to make things better but for that, you need to consider engagement as a key performance indicator, just like others.