Facebook post engagement rate measures how fans interact with the different Facebook posts of a brand.
Different formulas can be used for post engagement calculation. The engagement rate can be calculated for a single post or for all posts for a time frame.
A formula offered by Socialbaker:
The above formula has a limit, it does not really measure engagement of exposed fans, as only less than 20% of fans view a brand post in their newsfeed.
A more "wise" formula by Wisemetrics.
See also Facebook PTAT.