As indicated on https://developers.facebook.com/docs/reference/api/search/, http://graph.facebook.com/search?q=YOUR_QUERY is search all public posts, this kind of posts most likely not the post from your friends, normally you doesn't have permissions to comments.
If you want to search the post from your news feed, you can do
https://graph.facebook.com/me/home?q=a&access_token=USER_ACCESS_TOKEN
Example comment by post_id using curl:
<?php
//extract data from the post
extract($_POST);
//set POST variables
$url = 'https://graph.facebook.com/150552388343703_483757795023159/comments?access_token=USER_ACCESS_TOKEN';
$msg = 'hello world testing';
$fields = array(
'message' => urlencode($msg),
);
//url-ify the data for the POST
foreach($fields as $key=>$value) { $fields_string .= $key.'='.$value.'&'; }
rtrim($fields_string, '&');
//open connection
$ch = curl_init();
//set the url, number of POST vars, POST data
curl_setopt($ch,CURLOPT_URL, $url);
curl_setopt($ch,CURLOPT_POST, count($fields));
curl_setopt($ch,CURLOPT_POSTFIELDS, $fields_string);
//execute post
$result = curl_exec($ch);
//close connection
curl_close($ch);
?>