This pertains to older posts whose status that I have changed from published to draft and published again in order to add a facebook link.
On my site, admins and editors can see the WordPress field for post excerpt, but authors cannot. This may be default for WordPress; I cannot recall changing this in the role editor.
I have left the Add Link to Facebook setting "use excerpt as message" UNCHECKED.
Upon publish with "Do not add link to Facebook" UNCHECKED, the post excerpt nonetheless appears as message on the FB Page, but ONLY if the the post excerpt field on the edit post page is visible and contains text.
For authors, no message appears on the FB page, as should be, even if
post except field is not empty. That appear to be because authors cannot see the post except field.
The workaround for admin is to cut the post excerpt, then past it back once post is re-published with "Do not add link to Facebook" UNCHECKED