I recently had a fascinating discussion in a Social Media group about the 'correct' way to report Facebook Reach for Facebook posts and advertising. It's something that can be surprisingly complicated and I realised that since it's becoming the key KPI for most campaigns, I should share my thoughts on it.
The absolute basic definitions are usually:
Impressions: Number of views
Reach: Number of viewers
But this can often be misleading, as it doesn't take into account the full scenario.
What are you counting the Reach of and for how long?
You should be careful not to mix reach and impression terminology and try to be clear in your definition of Reach, as it could get incredibly complicated later on when trying to consistently report.
I find it helps to define if you want reach to be "unique" to content, time or campaign. I've started using the below terms.
Content Reach: Reach for each unique post accumulated. One person will be counted twice for seeing 2 different pieces of content but they won't be counted twice for seeing 1 piece of content 2 times.
Time Reach: Unique users in a period of time. For Page or Post level data this is only unique up-to-28 days. For Ads it's unique up-to any time you set.
Campaign Reach: Only relevant to ads. Unique to the campaign as a whole. 1 user counted, no matter how many ads seen in the campaign or how long it is.
Campaign and Time Reach can be quite difficult to estimate as costs for new unique users reached doesn't stay consistent, it curves upwards dramatically as you reach more people. Though the Reach & Frequency bid tool can help a little with this.
Personally I'm a fan of Content Reach, as it's the easiest to consistently report on across posts and ads, and it's easier for KPI estimation. It also takes into account the positive nature of a user seeing multiple different pieces of content, that may have different messages.
Content Reach is the metric I normally recommend using.