Today I realised that the customization in our Facebook share button was not working anymore. The reason is that Facebook changed it and it is not possible to just use the share.php method. You need a new one.
The former way is explained here http://ar.zu.my/how-to-really-customize-the-deprecated-facebook-sharer-dot-php/
But if you want to customize the caption and the text, you will have to get a Facebook App and use the sharer only from within the registered domain for that app.
It is explained to a great level of detail in this blog post: