The Facebook Insights view that shows a Page Administrator the number how many people have been exposed to a specific post.

Why does the Reach Tab matter to you?

The Reach Tab in Facebook Insights allows you to see which facets of your marketing strategy are most effective. You can compare the performance of your organic posts, paid advertisements, and viral content. This allows marketers to hone in on the best strategy for their company and improve their ROI.

Facebook Reach Tab on the Insights Dashboard