We’ve known that calculating reach and frequency metrics for Facebook is a tough engineering challenge, especially to do it in near real-time.
While Facebook has been vague as to which metrics are affected and to what extent, our guess is that the calculations of uniques by day and post are the main issue. Interactions, fans, and other aggregable metrics are easy to calculate– but deduping users across multiple sessions and devices is not.
Reach is really just unique impressions. And the sum of daily reach across a week is not the same as weekly reach, since people can see messages on multiple days. In the world of web analytics, we call this the non-aggregable metrics issue.
On February 25th, you can go into your analytics to see what’s changed.
Make sure you take a snapshot now, since they’re not going to keep the old, incorrect data for you.
Meanwhile, I’d rely upon other methods to analyze Facebook data via the graph API and ads data.