In order to post to Facebook, you must accept all of the Facebook posting permissions.
When posting a job listing to Facebook you will be prompted by Facebook to accept the public profile permission first:
They you will immediately be prompted to accept the pages permissions:
Note: You must be listed as an admin for your company’s business Facebook page.
Your Facebook must use your company Facebook ID which you can add to the settings in ApplicantStack by going to Setup--> Job Board --> Edit Settings [for Facebook].
If you do not know your Facebook ID, you can find it by using the following link: