As of this time, the Facebook Custom Audiences export job in Sailthru only exports email addresses. The reason is because this feature all of our clients use this feature, and custom variables differ across all clients. For example, the user's first name might be stored as "first_name" in your account, but "firstname" in another client's account.
As a workaround, we suggest:
Run a job export from your account on the lists you want to export. You can select "Select Data" and input which custom vars to include in the export.
From that you can see how closely that aligns with Facebook's fields you want for additional data.
With a custom script, push this list automatically to Facebook, making sure that your vars match Facebook's.
This is the step that won't be scaleable on Sailthru's side as that exists as the same code for all customers and not everyone will use the same custom variable names.