Engagement rate is calculated by dividing the Page's PTAT with the total number of Likes. This number represents the ratio between the amount of people talking about a brand and the number of people that actually follow that brand on Facebook. 

Engagements rates that are above 100% mean there are more people talking about the brand (PTAT) than there are followers of the page. For a definition of 'people talking about this' please read on here

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