What is the difference between impressions and reach?
Impressions and reach are terms used in Facebook advertising reports as metrics to measure campaign success. Understanding the subtle difference between the two terms is key in understanding how your ads are performing.
Impressions are the number of times your ad or post has been displayed. It is possible that one person could see the same ad multiple times in their news feed.
Reach is the number of people who received an impression of the ad or post. So it is possible that total reach could be fewer than total impressions if users have been displayed the ad multiple times.
For example: A user saw the same ad three times. Twice in their news feed and their friend shared the ad also. This scenario resulted in reach increasing by one and impressions by three.
Misapprehending impressions and reach is an easy mistake to make and the question has been asked by all fledgling Facebook advertisers at some point. All you need to remember is that impressions are the total number of times Facebook has displayed your content. Reach is the total number of users that saw that content.