I'm using the official Facebook plugin:
This is the site I'm working on:
This is the issue (notice popup box under facebook 'Send' button goes off-screen to the left):
NOTE: This problem only occurs when view on a mobile (tried both iPhone 4S and on a Windows phone), but it's very important that the site is also useable on a mobile. It looks fine enough in a web browser, and stays in its position even when the browser windown is narrowed.
(The facebook pop-up input box is however still too large).
I tried a lot of things in css, but nothing seemed to work or just made the Send button disappear.