Russell Allert said
6 months, 3 weeks ago: @cleelake Here is Facebook’s official line:
The reach you see when you promote your Page post is an estimate. Your promotion may be seen by more or less people than indicated originally and you’ll only be charged for the actual reach of your promotion.
Promoting a post that has been targeted to a specific location or language may also reduce your reach. The number of people you reach by promoting your post is in addition to the reach your post normally receives.