Okay I tested everything again and it appeared things weren't working fine because of a caching plugin. It works great now! For people experiencing similar problems in the future: Flush all caches before testing, always, after every change in the settings you made :)
@Markus: I got one feature request. Right now, when a visitor recommends a page on fb, the hidden content is already shown when the fb comment/post box opens, right after clicking the button. Especially for recommendation this is very bad, because people won't even post something to fb anymore if they see the content appear already after clicking the "recommend" button. This is the same for the like button, except then there is at least a like registered, but people will never post something after they see the content appear already.
So, my request is: Can you change this, so content appears AFTER someone POSTS TO FB OR CLOSES the comment/post box, in other words after someone clicks post to fb or close button and not after they click the like/recommend button?
Thanks in advance again!