// social.php plugin file
function google_callback() {
    $wp_session = WP_Session::get_instance();
    // Get Google data all happens here...
    $user['name']     = $userObj->givenName;
    $user['surname']  = $userObj->familyName;
    $user['email']    = $userObj->email;
    $user['verified'] = $userObj->verifiedEmail;
    $wp_session['registeruser'] = $user;
    $wp_session['registertype'] = 'Google';
    wp_redirect(home_url('social-register'));
}

// theme functions.php file
function build_registerform($args,$content=null) {
    $wp_session = WP_Session::get_instance();
    var_dump($wp_session);
}