On Facebook Insights UI there is a section called "Reach". Using page_impressions_unique of Insights API (which is supposed to carry the "Reach" value) I get a value which is 1/3 of the one appearing on UI. It seems they add some other metrics but it's not clear how to get that very same value using page metrics from Insights API. https://developers.facebook.com/docs/graph-api/reference/v2.8/insights
Does anyone have any clue about it?
Reach in Facebook Ads Manager does not equal page_impressions_unique or even contain it. page_impressions_unique is a form of non-advertising, organic reach for your page; namely people who visited your page based on non-ads activity on Facebook.
Reach in Ads Manager is the number of unique ad impressions that Facebook delivered to people.
To see a full list of insights-related fields in our UIs as they map to the API, see "Metrics Names and API Fields" here https://developers.facebook.com/docs/marketing-api/insights