Reach
The number of distinct accounts that saw a post at least once, however many times each of them saw it.
The word people mean when they ask how many people saw it. It counts accounts rather than views, so a post seen four times by the same person adds one to reach and four to impressions.
Instagram and Facebook split it into followers and non-followers, and that split is the most useful thing on the screen. Reach that is mostly non-followers means the content travelled. Reach that is entirely followers means it was shown to people who had already opted in.
It falls when posting frequency rises past what an audience will absorb, which is the opposite of what most posting schedules assume.
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