Re-reading your post, it sounds like you're having the opposite issue than the thread you were pointed to. By default, the Facebook Page plugin will grow to the width of the container it is in. The other user wanted to specifically prevent that behavior so that the width was always constant, regardless of how it was viewed (which would likely be very large on mobile views).
You seem to want the Page plugin to adapt to whatever div it is in. If that's the case, that's the default behavior and should automatically happen.
First, I'd suggest reading about all the parameters from the Page plugin documentation:
developers.facebook.com/docs/plugins/page-plugin
Specifically:
* 500px is the max width the plugin can be. If your column is larger than this, there will be white space.
* The 'adapt-width' feature will only grow the plugin to as large as the set width. So, in your case, it would only go to 400px max on desktop anyways.
* The minimum width that the Page plugin can shrink to is 180px. If, on mobile, the column is slimmer than that, you will have an over-run.
Those are the limitations of the Page plugin itself and we can't work around those. Please test again knowing that and see if you still think something is wrong. If so, please post the URL of a page where we can see the issue and we'll gladly investigate.
Thanks,
Alex