Facebook has said the Page Insights metrics had a bug in the system since May 2016 and was a miscalculation on the daily reach. The correction means that a de-duplicated 7-day summary in the dashboard for 7-day summary will show 33% lower on average, and 28-day will be 55% lower. No other fields in the summary are affected.
Clearly, anyone looking at those states would have seen higher numbers than had been achieved.
Additionally, we’re making an improvement to Page organic reach to match what we’ve done for paid reach. Reach counts will now be based on viewable impressions. On Pages, we’ve historically defined reach as a person refreshing their News Feed and the post being placed in their feed. For paid ads reports, we’ve moved to a stricter definition that only counts reach once the post enters the person’s screen (“viewable impressions”). With the stricter definition, we estimate that reported reach will be 20% lower on average. Facebook to Fix Metrics Bugs in Insights [newsroom.fb.com]
This bug is yet to be fixed, but Facebook said it'll be done in the next few weeks.
It's quite a comprehensive post from Facebook which details a number of metrics being clarified, including, better categorization, more consistent definitions, clarified calculations, and better descriptive names.
In addition, Facebook is retiring Interest Lists
which will result in a drop in the total number of followers for profiles that created interest lists or were featured in lists.
Another aspect of miscalculation is the "Referrals" in Analytics for Apps. Apparently, the clicks were being recorded for other sources, such as photos and video, and that referrals were overstated by 6%, on average. Facebook is working to fix this, although no indications were provided.
Time spent on Instant Articles was also being over reported by an average of 6 to 7% for over a year, and this has now been fixed.
Video measurement of completions was being under-reported and may result in an increase of around 35% in the count.