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FB Likebox checks if a URL is valid by using the drupal valid_url() function. Facebook pages may contain not only latin but greek letters too since this is supported by modern browsers. For urls with greek characters valid_url() returns false. As stated in valid_url documentation I also tried to drupal_urlencode() the $fb_url variable before executing valid_url() but this didn't work either. As a result, in order to create a facebook like box I had to comment out the if(valid_url($fb_url,TRUE)) section. It most probably isn't a FB Likebox issue but a Drupal issue but since it does affect this module, I thought I'd better write it here.