Facebook 'Send' window goes off-screen on mobile phones.

Hi Support.

I’m trying to use this official Facebook plugin.

This is the site I’m working on:
https://www.xotc.dk

LOGIN INFO REMOVED

The issue is the popup box under facebook ‘Send’ button goes off-screen to the left:
https://postimage.org/image/6ugn4e8u5/

This problem only occurs when viewed on a mobile (tried both iPhone 4S and on a Windows phone).
It’s very important that the site works on mobile.
It looks fine in a web browser, and stays in its position even when the browser windown is narrowed.
However, even in the browser the facebook pop-up input box becomes too large for the width on a mobile.
I tried a lot of things in WordPress’s custom css, but nothing seemed to work or it just made the Send button disappear.

Any help would be very much appreciated!

(FYI: I’m trying to ask this same question on the support forum, for the WordPress theme I’m using: https://www.kriesi.at/support/topic/facebook-send-window-goes-off-screen-on-mobile-phones?replies=1#post-87850)