Here is the css code to create a flexible width layout for Facebook Like Box/Activity Stream to use in responsive designs:

/*

Make the Facebook Like box responsive (fluid width)
https://developers.facebook.com/docs/reference/plugins/like-box/
*/

/* This element holds injected scripts inside iframes that in some cases may stretch layouts. So, we’re just hiding it. */
#fb-root {
display: none;
}

/* To fill the container and nothing else */
.fb_iframe_widget, .fb_iframe_widget span, .fb_iframe_widget span iframe[style] {
width: 100% !important;
}