Think about the main variables of a subscription list.
Those who like the brand, are the subscribers and the list size is count of followers. Brands don’t get special access to information about users. Essentially the followers are signing up to see some of the brands posts in their newsfeed.
The next classic subscription method is to know that the message was delivered. The “Reach” metric in side of FB speaks to that metric. It’s the number of unique people who have seen the brand’s post. Simple? Yep, sure is! This view is from Insights Analytics.