On June 28th, 2017, Facebook announced the removal of the ability to modify the Link Snippet for all social messages. You can review their announcement at Modifying Link Previews. These changes will become effective on July 18th, 2017. For that reason, any changes made inside of the Permalink box (Image below) will not reflect in Facebook Link Posts past this day. For more information on these changes, check out Facebook's Link Snippet Change. If you're using WordPress or Drupal, we recommend using tools like YoastSEO. For more information, check out this post by Yoast.