From Facebook Developer Wiki (FbDevWiki.com)
Query this table to obtain XIDs for fb:comments objects associated with an application ID.
Highlighted rows are indexable / searchable and one is usually required in the WHERE portion of the query.
|app_id||string||The application ID. You can specify more than one application ID.|
|xid||string||The external ID of the fb:comments object being queried.|
|count||int||The number of comments associated with an XID.|
|updated_time||int||A Unix timestamp indicating the most recent time a comment was made.|