My Facebook Custom Audience does not match the list count in Sailthru, why?
The discrepancy is due to Facebook matching.
Facebook only adds people it can match. When exporting a list from Sailthru to Facebook, Sailthru uses sha256 hashed emails and sends those hashed emails to Facebook. Then Facebook verifies if that user exists in Facebook and proceeds to add them to the custom audience.
If the user is not on Facebook they don't get added because Facebook does not know who they are.
It is not uncommon where end users signed up for a newsletter with a work email address but their Facebook account is a personal email address. It won't be able to find a match in those cases.