Previously, there was no way to tell how many of a post’s impressions came from existing fans or new visitors who hadn’t Liked the page. These new metrics will help page owners better understand who they’re reaching and how Facebook’s News Feed algorithm works. For example, posts that gets several Likes and shares have higher reach than those with less engagement. It has never been quite clear whether the reach is higher because friends of fans are seeing the activity in their feed, or if it’s because the engagement becomes a signal for Facebook to show the post to more fans than usual. The answer might be both, but now with actual numbers for fan impressions, page owners will be able to know with more certainty.

For now, these metrics are only available via the API, but they should be available in the insights export for “post level data” soon. [Update: The new metrics are now listed in post-level insights export as "Lifetime Post Reach by people who like your Page," "Lifetime Post Paid Impressions by people who have liked your Page," "Lifetime Post Paid Reach by people who like your Page" and  "Lifetime Post Stories by people who have liked your Page."]

We’ve heard that this information will also be presented visually to page owners, but the design hasn’t been finalized. We imagine Facebook will put these metrics in the insights dashboard or in the hover card that appears when admins mouse over the reach data below each item they’ve shared.