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;
}

subtítulos dos artigos no facebook com o excerpt
Filtro plugin https://wordpress.org/plugins/fb-instant-articles/

For those that stumbled on here through Google, I got this working with a slight variation:

add_filter( 'instant_articles_subtitle', 'hf_instant_articles_subtitle', 10, 2 );

function hf_instant_articles_subtitle( $subtitle, $this ) {
	$post_id = call_user_func( array( $this, 'get_the_id' ) );
	$excerpt = get_the_excerpt( $post_id );
	if ( trim( $excerpt ) != '' ) $subtitle = trim( $excerpt );
	return $subtitle;
}

Thank you!
It works for me ↓
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; }