How to connect to Twitter API using Powershell and follow parameter

I am getting the following error using Microsoft Azure Powershell to connect to Twitter API:

Exception calling "GetResponse" with "0" argument(s): The remote server returned an error: <401> Unauthorised." At line:1 char:1 + $response = $request.GetResponse() ; + CategoryInfo : NotSpecified: (:) [], MethodInvocationException + FullyQualifiedErrorId : WebException

The background is that I am starting with HDInsight and have followed the example at http://azure.microsoft.com/en-gb/documentation/articles/hdinsight-analyze-twitter-data/ which worked.

I am now redoing the first step (connecting to Twitter API) and trying to use a different field of the api ( Instead of using "track", I am using "follow" ) and the script is erroring.

The script I am running in Powershell is the following :

Write-Host "Set variables ..." -ForegroundColor Green
$storageAccountName = "mystorageaccountname"
$containerName = "containername"

$destBlobName = "tutorials/twitter/data/tweets.txt"

$followString = "1920771606"
$follow = [System.Uri]::EscapeDataString($followString);
$lineMax = 10  #about 3 minutes every 100 lines

$oauth_consumer_key = "mykey";
$oauth_consumer_secret = "mysecret";
$oauth_token = "myoauthtoken";
$oauth_token_secret = "my_oauth_secret";

Write-Host "Define the Azure storage connection string ..." -ForegroundColor Green
$storageAccountKey = get-azurestoragekey $storageAccountName | %{$_.Primary}
$storageConnectionString = "DefaultEndpointsProtocol=https;AccountName=$storageAccountName;AccountKey=$storageAccountKey"

Write-Host "Create block blob object ..." -ForegroundColor Green
$storageAccount = [Microsoft.WindowsAzure.Storage.CloudStorageAccount]::Parse($storageConnectionString)
$storageClient = $storageAccount.CreateCloudBlobClient();
$storageContainer = $storageClient.GetContainerReference($containerName)
$destBlob = $storageContainer.GetBlockBlobReference($destBlobName)

Write-Host "Define a MemoryStream and a StreamWriter for writing ..." -ForegroundColor Green
$memStream = New-Object System.IO.MemoryStream
$writeStream = New-Object System.IO.StreamWriter $memStream

Write-Host "Format oauth strings ..." -ForegroundColor Green
$oauth_nonce =     [System.Convert]::ToBase64String([System.Text.Encoding]::ASCII.GetBytes([System.DateTime]::Now.Ticks.ToString()));
$ts = [System.DateTime]::UtcNow - [System.DateTime]::ParseExact("01/01/1970", "dd/MM/yyyy", $null)
$oauth_timestamp = [System.Convert]::ToInt64($ts.TotalSeconds).ToString();

$signature = "POST&";
$signature += [System.Uri]::EscapeDataString("https://stream.twitter.com/1.1/statuses/filter.json") + "&";
$signature += [System.Uri]::EscapeDataString("oauth_consumer_key=" + $oauth_consumer_key + "&");
$signature += [System.Uri]::EscapeDataString("oauth_nonce=" + $oauth_nonce + "&"); 
$signature += [System.Uri]::EscapeDataString("oauth_signature_method=HMAC-SHA1&");
$signature += [System.Uri]::EscapeDataString("oauth_timestamp=" + $oauth_timestamp + "&");
$signature += [System.Uri]::EscapeDataString("oauth_token=" + $oauth_token + "&");
$signature += [System.Uri]::EscapeDataString("oauth_version=1.0&");
$signature += [System.Uri]::EscapeDataString("follow=" + $follow);

$signature_key = [System.Uri]::EscapeDataString($oauth_consumer_secret) + "&" + [System.Uri]::EscapeDataString($oauth_token_secret);

$hmacsha1 = new-object System.Security.Cryptography.HMACSHA1;
$hmacsha1.Key = [System.Text.Encoding]::ASCII.GetBytes($signature_key);
$oauth_signature = [System.Convert]::ToBase64String($hmacsha1.ComputeHash([System.Text.Encoding]::ASCII.GetBytes($signature)));

$oauth_authorization = 'OAuth ';
$oauth_authorization += 'oauth_consumer_key="' + [System.Uri]::EscapeDataString($oauth_consumer_key) + '",';
$oauth_authorization += 'oauth_nonce="' + [System.Uri]::EscapeDataString($oauth_nonce) + '",';
$oauth_authorization += 'oauth_signature="' + [System.Uri]::EscapeDataString($oauth_signature) + '",';
$oauth_authorization += 'oauth_signature_method="HMAC-SHA1",'
$oauth_authorization += 'oauth_timestamp="' + [System.Uri]::EscapeDataString($oauth_timestamp) + '",'
$oauth_authorization += 'oauth_token="' + [System.Uri]::EscapeDataString($oauth_token) + '",';
$oauth_authorization += 'oauth_version="1.0"';

$post_body = [System.Text.Encoding]::ASCII.GetBytes("follow=" + $follow);

Write-Host "Create HTTP web request ..." -ForegroundColor Green
[System.Net.HttpWebRequest] $request = [System.Net.WebRequest]::Create("https://stream.twitter.com/1.1/statuses/filter.json");
$request.Method = "POST";
$request.Headers.Add("Authorization", $oauth_authorization);
$request.ContentType = "application/x-www-form-urlencoded";
$body = $request.GetRequestStream();

$body.write($post_body, 0, $post_body.length);
$body.flush();
$body.close();
$response = $request.GetResponse() ;

Write-Host "Start stream reading ..." -ForegroundColor Green

$sReader = New-Object System.IO.StreamReader($response.GetResponseStream())

$inrec = $sReader.ReadLine()
$count = 0
while (($inrec -ne $null) -and ($count -le $lineMax))
{
if ($inrec -ne "")
{
    Write-Host $count.ToString() ", " -NoNewline -ForegroundColor Green
    if ($count%10 -eq 0){
        write-host " --- " -NoNewline
        Get-Date -Format hh:mm:sstt
    }

    $writeStream.WriteLine($inrec)
    $count ++
}

$inrec=$sReader.ReadLine()
}

Write-Host "Write to the destination blob ..." -ForegroundColor Green
$writeStream.Flush()
$memStream.Seek(0, "Begin")
$destBlob.UploadFromStream($memStream) 

$sReader.close()
Write-Host "Complete!" -ForegroundColor Green