It's working as expected. Your facebook iframe has static height. You can reduce it in inc/widget/facebook_widget.php
Or you can use any Facebook widget plugins, there are many of them with better configuration
i think its not working kindly see the gap difference i am talking about the gap between facebook widget & CE: Price Mover there is bit more gap i have applied your given code but seems like not working.
you can place this in theme options - general options - custom css
Please check the attached image. You can see the gap between Facebook widget & CE: Price Mover and the gap between Facebook widget & Rehub Tab.
So you can differentiate between consecutive widget gaps. So provide me the good solution here.
Link on page with issue (Optional): topbestof.com