To help Page owners better understand who their posts are reaching, Facebook has added four new metrics to its Insights API.

Previously, it wasn’t clear whether your reach was higher because friends of fans were seeing it, or if something signaled for Facebook to show the post to more fans than usual. This is due to the fact that there wasn’t a way for you to tell how many impressions came from fans versus non-fans.

This week, Facebook started offering the following new metrics: Post Impressions Fan, Post Impressions Fan Unique, Post Impressions Fan Paid, and Post Impressions Fan Paid Unique.

Post Impressions Fan is the number of impressions for a Page post among people who have Liked your Page. This can include multiple impressions from the same person. Post Impressions Fan Unique, however, focuses on the number of individuals who saw a Page post. Generally the first number will be higher than the second.

Post Impressions Fan Paid is the number of fan impressions that came as a result of an ad, Sponsored Story, or promoted post. Whereas Post Impressions Fan Paid Unique is the number of fans who saw the post within an ad, Sponsored Story, or promoted post.

Currently these metrics are only available through the API. We expect that Facebook will provide this information visually to Page owners, but the design hasn’t been finalized. Once pushed, it’s likely that the metrics will be available within the Insights dashboard or in the reach data of the shared post.

[Via: Inside Facebook, Image credit: Juan Fotografia Compacta]