I have the same problem. Look at this site (scroll down):
https://www.xotc.dk
I was forced to set the width to at least 450px in the Facebook plugin settings or else the pop-up windows would just get cut in the right side.
I need to be able to view the facebook window on mobile like an iPhone, and is disappointed to see that it doesn’t scale according to the screen, and there are no settings to control it.
After setting it to 450px so it isn’t cut, I can use View Source in Safari to edit <div class=”_5v4 _5v8″> under the iframe with css, and set the width down to 300px to see how it “should look” on mobile:
._5v4 {
width: 300px;
}
Sadly it doesn’t work if I add this to my wordpress css ??
I really hope Facebook will consider making their plugin compatible with smartphones, it should be a ‘must’ for them.