Post数据后跳转回错误信息

我用HttpWebRequest POST登录信息到人人网，抓包工具上看，自动跳转了3次然后返回了网页，而我写的程序跳完返回登录失败，我看了下也是跳了3次，结果说登录失败，我无法对自动跳转的内容处理，怎样才能获得正确的页面。大家可以也去试试            HttpWebRequest req = (HttpWebRequest)WebRequest.Create("http://www.renren.com/PLogin.do");
            req.Method = "POST";            string param = "email=邮箱&password=密码&origURL=http%3A%2F%2Fwww.renren.com%2FHome.do&domain=renren.com";
            byte[] bs = Encoding.ASCII.GetBytes(param);           using (Stream reqStream = req.GetRequestStream())
            {
                reqStream.Write(bs, 0, bs.Length);
             }  
             HttpWebResponse hwrs = (HttpWebResponse)req.GetResponse();