VB - API para Twitter

Senhores,

Boa noite.

Estou tentando montar um script pelo VB com API do twitter, porem me aparece que não tenho autorização - cod. 401.

por exemplo:

CLASSE:

Imports System.Net
Imports System.IO
Imports System.Configuration
Public Class Comunica
Public Shared Function FazRequisicaoTwitter(ByVal metodo As String, ByVal usuario As String, ByVal senha As String, ByVal url As String) _
As DataSet
Dim requisicao As HttpWebRequest
Dim resposta As WebResponse
Dim rd As StreamReader = Nothing
Dim ds As New DataSet()
Try
'monta a requisicao
requisicao = DirectCast(HttpWebRequest.Create(url), HttpWebRequest)
requisicao.Method = metodo
'verifica a autenticacao
requisicao.Credentials = New NetworkCredential(usuario, senha)
'obtem a resposta
resposta = requisicao.GetResponse()
rd = New StreamReader(resposta.GetResponseStream())
'le o xml e gera o dataset
ds.ReadXml(rd)
rd.Close()
'retorna o dataset
Return ds
Catch ex As Exception
Throw ex
Return Nothing
End Try
End Function
Private Shared Function FazRequisicaoTwitter(ByVal metodo As String, ByVal url As String) As String
'obtem as credenciais do usuário no arquivo de configuração
Dim usuario As String
Dim senha As String
usuario = "XXXXX"
senha = "YYYY"
'monta a requisição
Try
Dim requisicao As HttpWebRequest = DirectCast(HttpWebRequest.Create(url), HttpWebRequest)
requisicao.Method = metodo
requisicao.Credentials = New NetworkCredential(usuario, senha)
'obtem a resposta
Dim resposta As WebResponse = requisicao.GetResponse()
Dim rd As New StreamReader(resposta.GetResponseStream())
Dim responseString As String = rd.ReadToEnd()
rd.Close()
'retorna o xml
Return responseString
Catch ex As Exception
Throw ex
Return Nothing
End Try
End Function

Formulário:

Public Class apiTwitter

Private Sub btnPostsRecentes_Click_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnPostsRecentes_Click.Click

Try
Dim texto As String = txtMensagemTwitter.Text

 Dim url As String = String.Format("http://api.twitter.com/1/statuses/update.xml", texto)
 Comunica.FazRequisicaoTwitter("POST", "XXXX", "YYYY", url)
 MsgBox("Mensagem postada com sucesso")
 Catch ex As Exception
 MessageBox.Show("Ocorreu um erro durante a conexÆo como Twitter : " & ex.Message)
 End Try

 End Sub

 Private Sub btnPostar_Click_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnPostar_Click.Click

 Try
 Dim dstwitter As New DataSet
 dstwitter = Comunica.FazRequisicaoTwitter("GET", "XXXX", "YYYY", "http://api.twitter.com/1/statuses/user_timeline.xml")
 DataGridView1.DataSource = dstwitter.Tables("status")
 Catch ex As Exception
 MessageBox.Show("Ocorreu um erro durante a conexÆo como Twitter : " & ex.Message)
 End Try

 End Sub

End Class

.... alguem pode ajudar por favor?

Grato.