The last thing I'd like to mention in this comparison of Reach vs. Impressions is how different social media platforms see these metrics. Facebook describes Reach as the number of people who viewed your ads at least once. It's calculated as a sum of three metrics.
The first one is
Organic Reach, which is all the social media users you've been able to reach for free. The second one is
Paid Reach, which is users who saw your
paid ads or boosted posts. The last one is
Viral Reach - users who saw your post because their friends interacted with it.
As for Impressions, Facebook defines this metric as the number of times your advertisements were on screen.
Instagram defines Reach as the number of unique social media users who saw your post or story. And Instagram Impressions are the number of times they were viewed. As you can see, these metrics are similarly defined as those on Facebook.
Twitter doesn't track Reach but registers
Twitter Impressions any time a user sees one of your tweets.