Use WordPress post excerpt as Facebook Instant Article subtitle

excerpt-as-instant-article-subtitle.php

<?php
/* Instant Articles (https://wordpress.org/plugins/fb-instant-articles/) - Use post excerpt as subtitle */
add_filter( 'instant_articles_subtitle', 'hf_instant_articles_subtitle' );
function hf_instant_articles_subtitle( $subtitle ) {
	$excerpt = get_the_excerpt();
	if ( trim($excerpt)!='' ) $subtitle = trim($excerpt);
	return $subtitle;
}