do_action in header, add action later?

by

You must register callbacks before they are called (add_action() and add_filter() are registration functions, they remember what to do when).

Try the following in your functions.php or in your plugin:

add_action( 'topofthetop', 'load_FB_JS_SDK' );

function load_FB_JS_SDK()
{
    if ( is_singular() && have_comments() )
        someclass::FB_JS_SDK();
}