Like many other modern advertising platforms, Facebook reports impressions. However, one of the great features of Facebook is that it also reports Reach, a measure of how many people actually saw your content.
That can help you to determine the effectiveness of your advertising targeting and guide your decision making about the way an audience is reacting to the message you’re trying to share with them.
The measurement reports people who are reached on any property that is part of the Facebook platform. That means that it can be helpful when trying to determine how your content is received differently when you use different ad placements.