would this work? :
<?php $facebook_url = get_field('facebook' , 'option');
if( $facebook_url ):
echo '<a target="_blank" class="facebook_icon" href="'.$facebook_url.'"><img src="https://www.facebookbrand.com/img/assets/asset.f.logo.lg.png" alt="facebook-icon"></a>';
//change src of facebook to your server
endif; ?>
