Starting in September, IMEG and many other Facebook page administrators noted that page reach seemed to be trending sharply downward. After several months, Facebook has admitted that it's data on page reach and impressions has been flawed since September. This was because Facebook was updating it's mobile apps for iPhone and Android to make them run faster, and as a consequence, sent only enough data to the mobile apps to make them display the newsfeed correctly.
According to Techcrunch, as individual users upgraded to the new apps, their activity was not reported back to Facebook (and page administrators) making it seem that all of those mobile app users had dropped off the face of the earth. Content was delivered as usual, but reach and insights did not reflect that. Another second bug caused double counting of desktop newsfeed ads ( as both organic impressions and paid impressions). In short, the data has been unreliable for several months with the bugs just being found and fixed last week.
Starting this past Monday, February 25, Facebook reports the bugs have been fixed and correct data will be flowing into Page Insights. Some have reported that numbers appear to have soared since Monday. Facebook added further information in this post on their Facebook studio blog. We will monitor the situation and changes in reported data for all of our clients.