C# Networking: Handling Clients?

[C# Programming]

Ill try keep it short, Usually when I write networking applications, I spawn a new thread every time a client connects to the server. This works fine for my small time applications, but how do people who deploy large scale servers handle client communications? If you have thousands of users, for example a simple busy irc server, surely you cant spawn threads for all the users, what other methods are there?

Andy