Get links inside a DIV

You need to take the value of the href-attribute of the anchor element using HtmlAgilityPack's GetAttributeValue method. You could access the single anchor element by extracting directly the content of the parent blockcode element like this:

//div[@id='first-tweet-wrapper']/blockquote[@class='twitter-tweet']

Then fetch the single link inside. A possible solution could look like this (in this case the input is facebook, but works with microsoft too):

try
{           
    // download the html source
    var webClient = new WebClient();
    var source = webClient.DownloadString(@"https://discover.twitter.com/first-tweet?username=facebook#facebook");
    HtmlDocument doc = new HtmlDocument();
    doc.LoadHtml(source);

    var div = doc.DocumentNode.SelectSingleNode("//div[@id='first-tweet-wrapper']/blockquote[@class='twitter-tweet']");
    if (div != null)
    {
        // there is only one links
        var link = div.Descendants("a").FirstOrDefault();
        if (link != null)
        {
            // take the value of the attribute
            var href = link.GetAttributeValue("href", "");
            Console.WriteLine(href);
        }
    }
}
catch (Exception exception)
{
    Console.WriteLine(exception.Message);
}

The output is in this case:

https://twitter.com/facebook/statuses/936094700

Another possibility is to directly select the anchor element using XPath (like @har07 suggested):

    var xpath = @"//div[@id='first-tweet-wrapper']/blockquote[@class='twitter-tweet']/a";
    var link = doc.DocumentNode.SelectSingleNode(xpath);
    if (link != null)
    {
        // take the value of the href-attribute
        var href = link.GetAttributeValue("href", "");
        Console.WriteLine(href);
    }

The output is the same as above.