Retrieve posts from meta key

To use the LIKE comparison, try a string:

$args = array(
    'post_type'     => 'post',
    'post_status'   => 'publish',
    'meta_query'    => array(
        array(
            'key'   => $user_key,
            'value' => 'followed', // String here
            'compare'   => 'LIKE',
        ),
    ),
);

return  get_posts($args);

Also, you can use WP_Query to help you debug:

$args = array(
    'post_type'     => 'post',
    'post_status'   => 'publish',
    'meta_query'    => array(
        array(
            'key'   => $user_key,
            'value' => 'followed', // String here
            'compare'   => 'LIKE',
        ),
    ),
);

$query = new WP_Query( $args );

echo '<xmp>';
print_r( $query->result ); 
die;

$query->result should contain the generated SQL, and will show you how WordPress is preparing your meta data.