tl;dr: you can only visualize 1, 7, and 28 days periods when creating scorecards; when creating time-series you'll be able to visualize any time frame.
As for November 11 of 2021, we adjusted the reach-related metrics for our Facebook Insights connector to ensure accuracy in the data provided.
To give some context, the Facebook Insights API doesn't give the total reach of a selected period by default, but it gives periods of 1, 7, or 28 days.
Before, when you created a scorecard using Porter to visualize the last's month reach, you would actually get the last 28 days, showing a discrepancy. We did this to always deliver a metric, instead of an error.
The metric reach presents limitations for any reporting tool and across other social media APIs due to, possibly, it's a predicted metric or privacy issues. You'll find limitations of this metric on Twitter and LinkedIn, for instance. As such we suggest you to follow this guide and use the metric impressions instead.
Based on these changes, these are the new possibilities and limitations when reporting reach for Facebook Insights on Google Data Studio.
Scorecards
When visualizing the page total reach—or any reach metric— on a scorecard, you should always choose periods of 1, 7, or 28 days.
It's not just the last 1, 7, or 28 days, but whatever historical period. Here, for instance, I'm reporting 28 days from October 17 to November 13.
If you want to have scorecards and visualize any period of time, we encourage you to use the metric impressions instead of reach. Facebook will always provide this metric accurately regardless of the timeframe you choose.