Twitter share-link in Colorbox

I try to open a custom Twitter-Share-Link in Colorbox.

I tried:
<a class="colorbox-load" href="http://twitter.com/share?url=http%3A%2F%2Fwww.drupal.org%2F&amp;width=500&amp;height=500&amp;iframe=true">

and (without iframe):

<a class="colorbox-load" href="http://twitter.com/share?url=http%3A%2F%2Fwww.drupal.org%2F&amp;width=500&amp;height=500">

The Colorbox-window opens, but is empty.
Without iframe it shows "This content failed to load".

Is it possible to load such a link in a Colorbox?
If yes: how?

Thank you for any advice,
Tobias