i_like_facebook.user.js

// ==UserScript==
// @name           I like Facebook
// @namespace      http://d.hatena.ne.jp/youpy/
// @include        http://www.facebook.com/*
// ==/UserScript==

if(window.location.href.match(/facebook\.com\/(plugins|widgets)/)) {
  unsafeWindow.onloadRegister(function() {
    click(document.querySelector('a.like_button_no_like'));
  });
}

function click(e) {
  var evt = document.createEvent('MouseEvents');
  evt.initMouseEvent('click', true, true, window, 0, 0, 0, 0, 0, false, false, false, false, 0, null);
  evt.element = function (){ return evt.target; };
  e.dispatchEvent(evt);
}