Wordpress - 获取以下用户的用户列表

I'm using the plugin "user following system" and all I need to get is a list of users which I follow. I tried it with the snippet below but it always returns an array! What am I doing wrong?

function pwuf_get_following( $user_id = 0 ) {

    if ( empty( $user_id ) ) {
        $user_id = get_current_user_id();
    }

    $following = get_user_meta( $user_id, '_pwuf_following', true );

    if ( empty( $following ) ) {

    return;

    }
    return (array) apply_filters( 'pwuf_get_following', $following, $user_id );

ob_start(); ?> 
<ul>
    <?php
    $following = get_user_meta( $user_id, '_pwuf_following', true );
 foreach( $following as $user ) { ?>
        <li><?php echo $user->first_name; ?> <?php echo $user->last_name; ?></li>
   <?php } ?>
</ul>
 <?php

  return ob_get_clean();
}
add_shortcode( 'following', 'pwuf_get_following' );

</div>