Better yet, use C#/ASP.NET with SignalR. SignalR automagically negotiates the best underlying technology (whether that's long polling, SSE, Web Sockets, or whatever), and provides you with a nice little abstraction layer over the top. And it scales quite nicely.
You do realize that socket.io (node) did it first?
I'm all for SignalR and even like C#/ASP.Net (and MVC). That said, there are other factors to consider when developing/deploying an application in .Net ... the abstractions needed for testable C# are particularly annoying imho.
