What metric indicates the number of people who have seen a post on Facebook?

The correct answer is focused on the metric "Reach," which specifically measures the number of unique users who have seen a post on Facebook. This metric is crucial for understanding the potential audience exposure for a particular post or advertisement. Reach provides insight into how far content is spreading across Facebook, helping brands assess the effectiveness of their content in generating awareness among distinct individuals.

In contrast, engagement refers to interactions people have with the content, such as likes, shares, and comments, but it does not convey how many people have merely seen the post. Impressions track the total number of times a post or advertisement appears on users' screens, which could include multiple views by the same individual and does not reflect unique users. Clicks, on the other hand, indicate the number of times users have interacted with the post by clicking on a link or call-to-action, but again, this does not reveal the number of unique viewers. Thus, "Reach" is the most accurate measurement for understanding how many unique individuals have seen your Facebook post.