Block facebook's "read status" notification on chat. Your contact won't know if you read his message or not.

facebook-read-status.user.js

// ==UserScript==
// @name Facebook read status
// @description block facebook's "read status" notification on facebook chat.
// @namespace frs
// @include http://www.facebook.com/*
// @include https://www.facebook.com/*
// @grant none
// @version 1
// ==/UserScript==
 
(function() {
    var proxy = window.XMLHttpRequest.prototype.open;
    window.XMLHttpRequest.prototype.open = function() {
        if( arguments[1] !== '/ajax/mercury/change_read_status.php') {
            return proxy.apply(this, [].slice.call(arguments));
        }
    };
})();