Unfortunately, you can't. The Facebook Like Box doesn't allow for custom CSS to be applied. They used to allow more control over the looks, but the power was abused to make the module not look like Facebook at all. The only thing you could do is increase or decrease the width of the module to make another row of faces appear, which should use up more of that white-space.
We're working on our own 'stream' module, which will be included in JFBConnect v6.1.. due out tomorrow. At the bottom of this page, you can see the stream that's generated, which can be a combination stream of Facebook Page & Group, Twitter streams and LinkedIn Company posts. The output is completely overrideable and CSS stylable. RIght now though, it will not let you show the user faces like the Like Box module you show above.
I hope that helps answer your questions,
Alex
css to center facepile scsocial widget?
