facebook.com.user.js

// ==UserScript==
// @name         Hide FB Chat button
// @description  Hides FB chat button and sidebar
// @include      *.facebook.com/*
// @exclude      *.facebook.com/ajax/*
// @grant        GM_getValue
// @require      http://ajax.googleapis.com/ajax/libs/jquery/1/jquery.min.js
// ==/UserScript==
//
document.addEventListener('DOMSubtreeModified', function() {
  var button = document.getElementById('fbDockChatBuddylistNub');
  if (button) button.parentNode.removeChild(button);

  var sidebar = document.getElementById('pagelet_sidebar');
  if (sidebar) sidebar.parentNode.removeChild(sidebar);
});