If your website Custom Audience size is smaller than the number of website visits or pixel fires, it may be because:

• Only pixel fires get counted for website Custom Audiences.
• People viewing your website could've opted out or have other blocking tools enabled, so not every visit to your website will send a pixel fire to Facebook.
• Website visits may not always be matched to someone on Facebook. For example, if someone isn't logged into Facebook when they view your website, we can't add them to your audience.
• If a viewer visits web pages with a pixel 20 times, it'll fire 20 times, but it'll only be one unique person. Check the average number of pages viewed per person on your website. Similarly, if a viewer visits a page with multiple events (e.g. Key page view, add to basket), each triggered event will count as individual pixel fires but will only map to one person.
• Your audience may be limited by an audience rule. Check to see if this is the case.
• If a pixel is installed multiple times on a given web page, then multiple pixel fires only count as one valid person.
• Third-party analytics tools count unique sessions while Facebook counts people. If someone visits your website from their phone and desktop computer, or erases a cookie and refreshes the page, the third-party analytics will count two, whereas Facebook will count one.
• Time on which uniques is counted is often different. Third-party analytics tools often count uniques only once per 30 days, whereas Facebook counts uniques all the time.

Pixel fires and third-party analytics sites

Sometimes advertisers try to compare Facebook pixel fires with total visitors from other analytics platforms and see discrepancies. Here are some things to bear in mind when trying to compare numbers:

• Remember that one unique person can cause multiple pixel fires (e.g. PageView fires on every page a person visits). If your third-party site says there have been 20,000 visitors, check how many pages each visitor views on average. For example, if people view seven pages on average, you should expect about 140,000 pixel fires or more, given that you can have multiple pixel events firing on each page.
• You shouldn't expect numbers to perfectly line up between different platforms. You could try to have the pixel fire only once per person, but that would limit your retargeting and conversion tracking abilities. Think of these platforms as different, and use each for their strengths. Third-party analytics sites are great for understanding your website activity, while Facebook is great for understanding people, and allowing you to build Custom Audiences and then track and optimise for conversions.
• Remember that the number of pixel fires does not equal unique visitors. This is intentional, as firing the pixel on every page allows you to build audiences and custom conversions off specific pages that people visit.

* Nguồn: Facebook