The Facebook Page Plugin is semi-responsive. On page load, it will resize itself to the container that it's in. However, it does not dynamically resize as you resize the page.
Please test by resizing the page and then reloading the page, which should show the widget shrink or grow. Since mobile phones, and usually most users browsing your site, don't resize while viewing a page, this is the behavior that Facebook implemented.
For more information, see:
developers.facebook.com/docs/plugins/page-plugin
I hope that helps explain,
Alex