Facebook today revealed that bugs in its page insights product led reach and impressions to be misreported for months. Fixes are being rolled out today and over the weekend, the company says.

Facebook says the bugs only affected reporting, not delivery, so page reach wasn’t actually reduced, it may have just appeared that way from insights data. This was limited to page insights and did not have an impact on ad insights.

The social network says pages will be affected differently depending on their fan based and when and how frequently they post, but overall it expects most pages to see an increase in reach or no change. Pages that ran News Feed ads are likely to see an increase in paid reach. It’s possible some pages could see an increase or decrease in organic reach.

TechCrunch, which got details about the bugs, says one issue was the result of Facebook’s new mobile apps. In speeding up its native apps, the social network accidentally stopped counting how many people saw a page’s posts. A separate bug caused desktop News Feed ads to be counted as both organic and paid impressions.

Engagement rate and virality, which are metrics that are computed from reach and impressions, could also be affected.

Facebook says it will not be able to backfill page insights with accurate historical data. Instead, pages will have to look at their performance moving forward to estimate the impact, if any, on their pages previously. Facebook advises page owners to track their organic, paid and viral reach, as well as impressions for their page and posts over the next few weeks, beginning Monday.

Facebook discovered the bugs as part of an engineering audit of page insights. The company says it has put “a number of additional quality and verification measures” in place to avoid issues like this in the future.