The measurement is made by dividing the reach by the number of impressions. Your Facebook ad frequency would be 2 if your advertisement was viewed 10,000 times but only reached 5,000 individuals, indicating that on average, your advertisement was viewed twice.
Only when ad frequency reduces your results should you be concerned, and another time when a large number of people hide your advertisements because you are promoting too frequently on your Facebook custom audiences.
People frequently overlook Facebook ad frequency as a statistic, and while some should, others should not.
Facebook ad frequency can destroy your retargeting efforts by degrading your results or, worse still, by causing individuals to block your advertisements, which prevents you from reaching them any longer. The latter could shrink your retargeting audience and prevent you from showing ads to a group of people you’ve worked to develop.
The solution is to determine the appropriate reach and effective frequency for each campaign, which can only be accomplished through testing and optimization. Your initial reach and frequency should be strongly reliant on research to establish the criteria of what you believe would be the ideal number.