If you use a third-party reporting tool to gather insights about your ads, your reporting may not match Facebook ads reporting.

The following are ways that Facebook ads reporting conversion counts may differ with a third-party reporting tool:

• Use of people-based measurement: Third-party reporting platforms aren't able to measure cross-device conversions well due to cookie-based measurement. Mobile conversions tend to be under-reported by third-party platforms that measure with cookies. Facebook conversion tracking allows you to understand people's behaviour across devices through the use of cross-device reports.
• Ability to attribute conversions to impressions: If a person sees a Facebook ad for your product and doesn't click, but later browses your website and decides to purchase, Facebook is able to attribute this conversion to the Facebook ad that the person saw. Third-party platforms are unable to capture these behaviours.
• Shows conversions based on time of impression: Facebook shows conversions in your reports based on the time of impression and not the time of conversion. For example, if someone sees your ad and then buys a product on your website a few days later, we show this purchase based on the time that someone saw your ad and not when they purchased your product. Depending on your attribution window, your results in Facebook ads reporting may differ from third-party reporting platforms, which show results based on when someone converted (e.g. purchased an item, added a product to a shopping basket or downloaded an app), and not when they saw the ad.

* Nguồn: Facebook