From Facebook Developer Wiki (FbDevWiki.com)
Query this table to obtain comments associated with one or more fb:comments/Feed story comment XIDs.
Highlighted rows are indexable / searchable and one is usually required in the WHERE portion of the query.
|xid||string||The external ID of the fb:comments being queried. You can specify more than one XID.|
|object_id||string||The object_id of an object on Facebook. This can be a video, note, link, photo, or photo album. Note that for photos and albums, the object_id must be queried from the photo and album FQL tables. Note that in the photo and album tables, object_id is a different field from pid and aid. You must specify either an xid or an object_id.|
|post_id||string||The ID of the post in the stream.|
|fromid||int||The user submitting a comment.|
|time||int||A Unix timestamp associated with the creation time of a comment.|
|text||string||The text of a comment.|
|id||int||A unique ID for a given XID for each comment.|
|username||string|| The user name that a user entered when they posted a comment. In these cases, the |
|reply_xid||string||The target XID for Feed stories generated by the user; applications can retrieve comments made to that story by calling this XID.|