Donors were previously invited to opt in when they donate to your organization.
You can find all donors who had previously opted in to hear more from you by following these steps:
Head to the "Audience" tab in the main navigation bar at the top of your account and then select "Donations" from the drop-down
Expand ''Filters''
Select "Other"
Select ''only with opt-in consent''
This will show you donors who, when donating on Facebook, opted in.