custompress custom field IF !empty

I want to display four custom fields, if they have a value set in the back. I need to make an IF statement to check to see if a variable is empty or not, but variables arn't used with this plugin, shortcodes are... So how do I see what variable is associated with the shortcode.

Like if (!empty($facebook)) { customfieldshortcode}

The reason for this, is each shortcode outputs a url, that is in a anchor tag with a img. I dont want the img to show up if there is no value in the input field (shortcode).

<?php

/** Replace the standard loop with our custom loop */
remove_action( 'genesis_loop', 'genesis_do_loop' );
add_action( 'genesis_loop', 'child_do_custom_loop' );

function child_do_custom_loop() {

if(have_posts()) :
while(have_posts()) :
the_post();
?>

<div class="agent">
<?php if ( has_post_thumbnail() ){ ?>

<div class="agent-image"> <?php the_post_thumbnail(); ?> </div>

<?php } else { ?>

<div class="agent-image">

<img src="http://pensacolamainstreet.com/msp/wp-content/uploads/sites/4/2013/09/Main-Street-Max.jpg" />

</div>

<?php } ?>

<div class="agent-title">

<h2> <?php the_title() ?> </h2>

</div>

<div class="agent-social">

I have four custom fields, facebook, linkedin, twitter, and Google+. I want to display each one here, IF it has a value.

</div>

</div>

<?php
endwhile;
else :
?>

Oops, there are no posts.

<?php
endif;

}