According to the code samples in the article _user_liked
post meta should hold all the user ids of those who liked it. Something like this should be able to solve it for you
<?php
$post_id = get_the_ID();
$users_liked = get_post_meta( $post_id, '_user_liked', true );
if ( '' !== $users_liked && ! empty( $users_liked ) ) {
?>
<ul>
<?php foreach ( array_values( $users_liked ) as $user_id ) :
$user = get_user_by( 'id', $user_id );
if ( false === $user ) {
continue;
}
?>
<li><?php echo $user->user_login; ?></li>
<?php endforeach; ?>
</ul>
<?php
}