All you do with any Web site or application is sending some HTTP requests and receiving HTTP responses. This is how you get information, post data, authenticate, anything.
You can use the class
System.Net.HttpWebRequest
:
http://msdn.microsoft.com/en-us/library/system.net.httpwebrequest.aspx[
^].
Make sure you don't do anything illegal, abusive or ethically questionable (like spamming), which is always a concern if you try to represent some user through your application — please see the comment by PIEBALDconsult about a "bot".
—SA