Programmatically adding widgets to custom WordPress sidebar

I’m working on a WordPress plugin. I can add my three custom widgets and a dynamic sidebar to the interface. I’m trying to set it up to load some default widgets into the sidebar if the user hasn’t set up any.

function rs_sidebar_setup() {
    // register sidebar
    $rs_sidebar_opts = array(
        'name' => __('Social Media Updates'),
        'id' => 'social-updates',
        'description' => __('Displays recent posts from social media sites'),
        'before_widget' => '',
        'after_widget' => ''
    );
    register_sidebar($rs_sidebar_opts);
}
add_action('widgets_init', 'rs_sidebar_setup');

if ( !function_exists('rs_default_widgets') ) :
function rs_default_widgets() {
    // get active widgets from database
    $widgets = get_option('sidebars_widgets');

    //if(empty($widgets['social-updates'])) {
        // add new widgets to our sidebar if the sidebar is empty
        //$widgets['sidebar-social-updates'] = array('calendar-3', 'rapidsocialfacebookwidget-1', 'rapidsocialfwitterwidget-1', 'rapidsociallinkedinwidget-1');
        $widgets['social-updates'] = array('calendar-3');
        update_option('sidebars_widgets', $widgets);

    //}

    print_r(get_option('sidebars_widgets'));
}
add_action('widgets_init', 'rs_default_widgets', 11);
endif;

In case it was a problem with my custom widgets, I tried using built-in widgets – still no good. The print_r statement confirms that the database is changed, but I still don’t see a calendar widget in my Social Media Updates sidebar.

I tried using the same get_option/update-option technique with a sidebar built into the Twenty Eleven theme, and it worked, so I’m thinking it might be a problem with the load order, but I don’t know. Any thoughts?