As of 30 March 2013, the most comments on a Facebook item is 15,629,405, achieved by Guru Shri Rajendraji Maharaj Jaap Club (India), in response to a post made on 27 March 2012.
The post can be seen here: https://www.facebook.com/permalink.php?story_fbid=406176996060953&id=100000059694577
Please note: Comments below may relate to previous holders of this record.