Store custom fields in Site Information Settings per domain.

Hello,
I created a module that hooks to the forms system_site_information_settings. My objective is to store additional fields per domain; the module works ok with one domain however, when I submit the form the values are set equally for all domains. How can I store individual values for each domain? Below is the module I created:

/**
* @filesource
* mysiteinfo.module
*/

function mysiteinfo_form_alter(&$form, &$form_state, $form_id) {
  if ($form_id == 'system_site_information_settings') {
    $form['mysiteinfo_extra_setting2'] = array(
      '#type' => 'textfield',
      '#title' => t('Facebook'),
       '#default_value' => variable_get('mysiteinfo_extra_setting2'),
    );

    $form['mysiteinfo_extra_setting3'] = array(
      '#type' => 'textfield',
      '#title' => t('Twitter'),
    '#default_value' => variable_get('mysiteinfo_extra_setting3'),
    );

    $form['mysiteinfo_extra_setting4'] = array(
      '#type' => 'textfield',
      '#title' => t('Google'),
         '#default_value' => variable_get('mysiteinfo_extra_setting4'),
    );

    $form['mysiteinfo_extra_setting5'] = array(
      '#type' => 'textfield',
      '#title' => t('Pinterest'),
         '#default_value' => variable_get('mysiteinfo_extra_setting5'),
    );
    $form['mysiteinfo_extra_setting6'] = array(
      '#type' => 'textfield',
      '#title' => t('RSS'),
         '#default_value' => variable_get('mysiteinfo_extra_setting6'),
    );
  }

// extra submit form handle
  $form['#submit'][] = 'mysiteinfo_form_submit';
}


// handle form submition 

function mysiteinfo_form_submit($form, &$form_state){
    
    $values = $form_state['values'];
    
    if(!empty($values['mysiteinfo_extra_setting2'])){
        variable_set('mysiteinfo_extra_setting2', $values['mysiteinfo_extra_setting2']);
    }
    
    if(!empty($values['mysiteinfo_extra_setting3'])){
        variable_set('mysiteinfo_extra_setting3', $values['mysiteinfo_extra_setting3']);
    }

    if(!empty($values['mysiteinfo_extra_setting4'])){
        variable_set('mysiteinfo_extra_setting3', $values['mysiteinfo_extra_setting4']);
    }

    if(!empty($values['mysiteinfo_extra_setting5'])){
        variable_set('mysiteinfo_extra_setting3', $values['mysiteinfo_extra_setting5']);
    }

    if(!empty($values['mysiteinfo_extra_setting6'])){
        variable_set('mysiteinfo_extra_setting3', $values['mysiteinfo_extra_setting6']);
    }
}