This is how I do it, at the time of the post...
FB.Event.subscribe('comment.create', function (response) {
var commentQuery = FB.Data.query('SELECT fromid, text FROM comment WHERE post_fbid=\'' + response.commentID + '\' AND object_id IN (SELECT comments_fbid FROM link_stat WHERE url=\'' + response.href + '\')');
FB.Data.waitOn([commentQuery], function () {
text = commentQuery.value[0].text;
fromid = commentQuery.value[0].fromid;
$.ajax({
type: "POST", // form method
url: "add_fb_comments.php", // destination
data: "&fromid=" + fromid + "&comment=" + text + "&commentID=" + response.commentID, // data to post
cache: false
});
});
});
Then in add_fb_comments.php
, simply save the details posted there.