https://developers.facebook.com/docs/pages-api/comments-mentions/

Get comments

To get the comments for a Page post, send a GET request to the /page_post_id/comments endpoint with the fields parameter set to a comma-separated list that includes the message field, to get the content for the comment and the from field, to get the Page-scoped ID (PSID) for the person or Page who commented on the post, if you would like to @mention the person or Page in the comment.

Example Request

Formatted for readability. Replace bold, italics values, such as page_post_id, with your values.

curl -i -X GET "https://graph.facebook.com/page_post_id/comments?fields=from,message"

On success, your app receives the following JSON response with the commentor's name, PSID, message and the comment ID:

{
  "data": [
    {
      "created_time": "2020-02-19T23:05:53+0000",
      "from": {
        "name": "commentor_name",
        "id": "commentor_PSID"
      },
      "message": "comment_content",
     "id": "comment_id"
    }
  ],
  "paging": {
    "cursors": {
      "before": "MQZDZD",
      "after": "MQZDZD"
    }
  } 

}  

https://developers.facebook.com/docs/pages-api/comments-mentions/