Simple add_shortcode code doesn't work

my wp plugin code:

<?php
/*
Plugin Name: ...
...
*/

class Pform {   
    function __construct() {
        add_shortcode('pform', array($this, 'show_form');
    }

    function show_form() {
        $html = <<<EOT
            <form id="pform" action="#">
            ...
            </form>
EOT;
        return $html;
    }   
}

function efinstall() {

    $tb_name = $wpdb->prefix . 'table';

    if($wpdb->get_var("show tables like '$tb_name'") != $tb_name) 
    {
       $sql = "CREATE TABLE " . $tb_name . " (
       id mediumint(9) NOT NULL AUTO_INCREMENT,
       ...
       UNIQUE KEY id (id)
       );";

       require_once(ABSPATH . 'wp-admin/includes/upgrade.php');
       dbDelta($sql);
       $form = new Pform();     
   }    
}
register_activation_hook(__FILE__,'efinstall');
?>

After plugin activation db table is successfully created and no error is added to log, but [pform] shortcode doesn't work and there is no it in

<?php
global $shortcode_tags;
echo "<pre>"; print_r($shortcode_tags); echo "</pre>";
?>

output.

Any help would be appreciated!

via Chebli Mohamed