Conditionally loading Facebook PHP SDK in shortcode

Since 3.3 we can enqueue scripts conditionally right in our shortcode functions, but when I tried to do this with some PHP class (uses session_start() in the __construct() function) as you can guess it gives me the headers already sent error.

The problem is (this is using the Facebook PHP SDK in conjunction with the JS SDK), I only want this class to be instantiated if a shortcode is present on the post and if the current post has 2 meta fields filled out already (the class needs these 2 values [App ID and App Secret]).

Is there a simple solution to this? If so, what can I change in the following code to allow this?

public function fb_connect_shortcode( $atts, $content = null ) {
    extract( shortcode_atts( array( ), $atts ) ); /**/ global $post;
    $app_id = get_post_meta( $post->ID, $this->meta_prefix . 'AppID', true );
    $app_secret = get_post_meta( $post->ID, $this->meta_prefix . 'AppSecret', true );

    if( $app_id !== '' && $app_secret !== '' ) {

        /**
         * This is the class I am trying to instantiate conditionally
         * but since it uses session_start(), it can't send out headers
         */
        $facebook = new Facebook( array(
          'appId'  => $app_id,
          'secret' => $app_secret,
        ) );

        // See if there is a user from a cookie
        $user = $facebook->getUser();

        if( $user ) {
            try {
                // Proceed knowing you have a logged in user who's authenticated.
                $user_profile = $facebook->api( '/me' );
                $app_id = $facebook->getAppId();
            } catch( FacebookApiException $e ) {
                echo '<pre>' . htmlspecialchars( print_r( $e, true ) ) . '</pre>';
                $user = null;
            }
        }

        wp_enqueue_script( 'jw-fbsdk', plugins_url( 'jw-fbsdk.js', __FILE__ ), array(), '3.3.1', true );
        wp_localize_script( 'jw-fbsdk', 'jwuc', array(
            'appId' => $app_id,
            'channelUrl' => plugins_url( 'channel.php', __FILE__ )
        ) );

        if( is_null( $content ) )
            $content="Connect with Facebook";

        if( isset( $user_profile ) && $user_profile ) {
            return $user_profile['name'];
        } else {
            return '<div class="fb-login-button" data-scope="email,publish_stream,read_stream,status_update">' . $content . '</div>';
        }

    } else {

        return "You forgot to add your App ID and/or App Secret! Facebook needs these. :)";

    }

}

You can try to use the_posts filter to search for you shortcode and require the sdk, something like this:

function has_my_FB_shortcode($posts) {
    if ( empty($posts) )
        return $posts;
    $found = false;
    foreach ($posts as $post) {
        if ( stripos($post->post_content, '[my_shortcode') ){
            $found = true;
            break;
        }
    }

    if ($found)
        require('path/to/facebook_sdk.php');

    return $posts;
}
add_action('the_posts', 'has_my_FB_shortcode');