The first and most important thing is to understand what Reach is.
It's defined by unique users who saw your content, but let's be more precise:
Page Reach = The number of unique people who had any content from your Page or about your Page enter their screen. This includes posts, stories, check-ins, ads, social information from people who interact with your Page, and more – during a specific period of time.
Post Reach = The number of unique people who saw a specific post on their timeline.
That's why Facebook gives you numbers for specific periods – because it filters only the unique users for every day, 7 days, or 28 days.
You can see the Last 28 Page Reach (applies for other ones as well) by placing the cursor on a specific date inside the graph:
In this case, you see that the reach between September 20 and October 17 is 25 316 unique users.
This implies that you cannot count and add on the data of each day, or even weeks, together. That is why Facebook gives the data automatically – Daily, Weekly, and for the Last 28 Reach – because the data shows unique users who saw your content for each of these periods.
Sometimes people want to know the Reach not for the last 28 days, but for a specific period, like October for example. And very often they do the mistake of looking at the graph and counting by adding up numbers for each day – then they end up with some crazy numbers.
The main problem is that one unique person, let's call him John, who came to your page every day for the last month, would be counted not only once, but 31 times. So, there is no such thing as "cumulative" reach 😉
To avoid this mistake, use your cursor inside the graph to see the data for the previous period of time.
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