facebookcom_notify_chang.user.js

// ==UserScript==
// @name           Facebook.com: notify changes of online friends
// @namespace      http://buycheapviagraonlinenow.com/
// @include        http://www.facebook.com/
// ==/UserScript==

var notifyServerUrl = 'http://localhost:5678/', // https://gist.github.com/1021283
    current, previous = [];

setInterval(function() {
  current = Array.map(document.querySelectorAll('.active'), function(item) {
    return item.textContent;
  });

  notify(only(current, previous), 'online');
  notify(only(previous, current), 'offline');

  previous = current.concat();
}, 5000);

function only(a, b) {
  var result = [];

  a.forEach(function(item) {
    if(b.indexOf(item) == -1) {
      result.push(item);
    }
  });

  return result;
}

function notify(names, behavior) {
  if(names.length > 0) {
    var msg = [names.join(', '), names.length > 1 ? 'are' : 'is', behavior].join(' ');

    document.createElement('img')
      .src = notifyServerUrl + '?text=' + 'Facebook: ' + encodeURIComponent(msg);
  }
}