Data is only shared to a facebook URL destination after a user has first qualified for a segment and then forces a secondary pageload where AAM DIL/AppMeasurement or Visitor ID are enabled.
For starters, segments will have varying “match rates” depending on the makeup of the segment. Segments comprised of site data should have a decent match rate; segments of offsite (media) or offline (CRM) data should be notably lower.
Below are some external docs from facebook which talks on improving the match rate.
1. Data share between AAM and FB involves a unique identifier at both ends which in general scenario is the deviceID. for AAM, it is AAM ID and similarly for Facebook it is this Id which gets identified. As customerIds are linked with AAMUUID only, using this information a unique customer can be identified.