Impressions

Always higher than reach, and the gap between the two is the interesting part. Impressions divided by reach gives frequency: how many times the average person who saw the post, saw it.

On organic content a frequency near one is normal and healthy. On paid it climbs with budget, and past roughly three or four in a week the same audience is being shown the same thing until it stops working.

Impressions are the easiest number on the screen to grow and the easiest to mistake for progress, which is why they belong beside reach rather than in front of it.