facebook background

It must be because of the CSS of your template as the background color IS transparent by default.
Please give the URL of that page so that we can see what CSS is the problem.

The problem comes from the code:
iframe {
background: #DADADA;
}

in the file polyedar.com/templates/gk_pixellove/css/light.css of your template.

You should change it to:
iframe {
background: #FFFFFF;
}