function fb_share() {
  var u = encodeURIComponent(location.href);
  var t = encodeURIComponent(document.title);
  window.open('http://www.facebook.com/sharer.php?u='+u+'&t='+t, 'sharer', 'toolbar=0,status=0,width=626,height=436');
  return false;
}

document.observe('dom:loaded', function () {
  $$('a.fb_share').invoke('observe', 'click', function (e) {
    e.stop();
    return fb_share();
  });
});