The remote server returned an error: (400) Bad Request. When try to login with facebook in asp.net

The remote server returned an error: (400) Bad Request. When try to login
with facebook in asp.net

I am try to login with a user with Facebook account in my website, but the
applcation gives me error that The remote server returned an error: (400)
Bad Request.
Below is my code:
public string WebRequest(Method method, string url, string postData)
{
HttpWebRequest webRequest = null;
StreamWriter requestWriter = null;
string responseData = "";
webRequest = System.Net.WebRequest.Create(url) as HttpWebRequest;
webRequest.Method = method.ToString();
webRequest.ServicePoint.Expect100Continue = false;
webRequest.UserAgent = "[You user agent]";
webRequest.Timeout = 50000;
if (method == Method.POST)
{
webRequest.ContentType = "application/x-www-form-urlencoded";
//POST the data.
requestWriter = new StreamWriter(webRequest.GetRequestStream());
try
{
requestWriter.Write(postData);
}
catch
{
throw;
}
finally
{
requestWriter.Close();
requestWriter = null;
}
}
responseData = WebResponseGet(webRequest);
webRequest = null;
return responseData;
}
public string WebResponseGet(HttpWebRequest webRequest) { StreamReader
responseReader = null; string responseData = "";
try
{
responseReader = new
StreamReader(webRequest.GetResponse().GetResponseStream());
responseData = responseReader.ReadToEnd();
}
catch
{
throw;
}
finally
{
webRequest.GetResponse().GetResponseStream().Close();
responseReader.Close();
responseReader = null;
}
return responseData;
}