Definition: Post shares are the number of times Facebook ads were shared on Facebook. Posts can be shared on a user’s own or friends’ Timelines, in groups or on Pages.
How it’s used: Post shares is a count of how many shares your ads received while running. It may also include shares on Instagram which were sent to people’s inboxes.
Post shares does not count engagement with the post after it is shared on another Page’s or person’s timeline.