get_theme_mod always returns default

In the Customizer code you need to do following change to work. You dont need ta_pluton_social_link[facebook] like array in Control. Check following example and modify accordingly to other fields also.

  $wp_customize->add_control('facebook', array(
      'label'      => __('Facebook URL', 'ta_pluton'),
      'section'    => 'ta_pluton_social_links',
      'settings'   => 'ta_pluton_social_link[facebook]',
  ));

To fetch URL you can use get_theme_mod. Check example below. Following snippet demonstrates fetching Facebook URL.

$ta_pluton_social_link = get_theme_mod( 'ta_pluton_social_link' );
$facebook_url = $ta_pluton_social_link['facebook'];