Just because this issue killed some hours of my precious time I would like to share these possible solutions with you.
Issue: Facebook like box does not appear in either google chrome, safari, internet explorer or whatsoever browser.
Answering my own question
Possible reasons: First of all keep in mind at all times that it might have something to do with your facebook login state in a browser.
Usually you are using a certain browser when surfing and developing. In this browser your are most likely logged into Facebook. When trying a different browser you may not be logged in and Facebook possibly hides the like-box from you if:
Your Facebook fanpage has country restrictions
Your Facebook fanpage has age restrictions and Facebook does not know your age when you are not logged in
Your Facebook fanpage has been created but it has not been published yet (This was my case and it almost drove me nuts.)
You can adjust these settings in the admin panel of your fanpage.
Happy coding :)