How does replace work in my lists?
When you upload a new list of contacts, you can choose to either add or replace. If you choose to replace contacts, there is information you should know.
When you add a new list of contacts, you have two options: Add new records and Replace exisiting records. If you choose to replace existing records, your current records are deleted and only contacts in the list you just uploaded are retained in Social Audiences. If you want any contacts from your existing records to be retained after you choose to replace existing records, ensure those contacts are in the new list you add.
Specifically for Facebook, you can add an unlimited number of records to an audience, but only a maximum of 10000 at a time. It may take up to 1 hour before Facebook adds users to an audience, and up to 24 hours for deleting users from an audience.
Learn more about Facebook Custom Audiences https://developers.facebook.com/docs/marketing-api/audiences-api