I'm not sure. Again, I see it working on your site. I haven't seen it 'not' work, though that's not to say there aren't issues.
If you look at your HTML versus the code that the Facebook page generates, they're the same (though we set the width, and a few other parameters). There's nothing wrong, that I can see, in your code. It's possible, though again unlikely, that it's possibly the different language file your importing (NL_nl vs EN_us) on the Facebook page you link to.
Can you try:
* Logging in with a different Facebook user and see if it's always blank? I don't know why it would be user dependent, but it's a thought.
* Temporarily setting your Language in Joomla to a different one to see if a different Facebook Javascript library affects the module?
Thanks