There are two types of data about your Custom Audience from your website that can be sent to Facebook:

• Contextual data: This type of data includes the referral URL, browser information and the person's Facebook user ID.
• Custom data: Websites can choose to send this data to Facebook through the Facebook Tag API. Bear in mind that the Custom Audience Terms of Service restrict anyone from using this feature to capture sensitive information about people using your website.

This information is shared in a privacy-safe way using hashing.

* Nguồn: Facebook