Page Impressions are the number of times a post from your Page is displayed in the Feed, whether the post is clicked or not. People may see multiple impressions of the same post. For example, someone might see a Page update in Feed once, and then a second time if their friend shares it.
The graph reveals the truth concerning how many times your content was seen. You can see a breakdown of organic, viral and paid impressions. They are defined as follows:
Organic impressions: The number of times any content from your Page or about your Page entered a person’s screen through unpaid distribution. This includes posts, check-ins, social information from people who interact with your Page and more. (Total Count)
Viral impressions: The number of times any content from your Page or about your Page entered a person’s screen with social information attached. Social information displays when a person’s friend interacted with your Page or post. This includes when someone’s friend likes or follows your Page, engages with a post, shares a photo of your Page and checks into your Page. (Total Count)
Paid impressions: The number of times any content from your Page or about your Page entered a person’s screen through paid distribution such as an ad. (Total Count)
Note: Based on the data Facebook provides, paid impressions are calculated for any content shared via paid distribution, meaning it could be boosted post, ad post, unpublished post, etc.