The top referring external domains sending traffic to your page. (Note: As of March 14, 2024, the Facebook API deprecated this data.)
Referrals: The number of referrals to your Page from external domains.
Sources: The number of external domains referring users to your Page.
For more information on Facebook metrics, please refer to the following FB developer resource: https://developers.facebook.com/docs/graph-api/reference/v20.0/insights