Cannot select signup_meta upon multisite user activation

Alright, finally I sorted it out – and it was a query error (embarrassing it is).

Correct code:

add_action( 'wpmu_activate_user', 'promote_to_admin', 99 );
function promote_to_admin( $user_id, $password, $meta ) {
    global $wpdb;
    $user = get_userdata( $user_id );
    $email = $user->user_email;
    // You need to prepare query first to avoid SQL errors
    $query = $wpdb->prepare( "SELECT meta from `wp_signups` where `user_email` = %s", $email );
    // By default, Object(stdClass) will be returned; 
    // tell WordPress to convert it to multidimensional array
    $raw_meta = $wpdb->get_results( $query, ARRAY_A );
    // After conversion, array will be represented as serialized string - unserialize it
    $signup_meta = unserialize( $raw_meta[0]['meta'] );
    if ( array_key_exists( 'new_role', $signup_meta ) ) {
        $user->set_role( $signup_meta['new_role'] );
    }
}