Facebook Page Insights will include new data around your viral content in the next few days.
Your viral content is the content you’ve posted on your Page that has received engagement and/or reach beyond your fans or friends of fans.
According to Facebook:
If someone shares a page post, we’ll attribute impressions of those shares to impressions from the original page post. We’ll also be capturing page post impressions on an ongoing basis. Previously, if a post received impressions after 28 days, those numbers weren’t captured.