SignalR is the .NET equivalent of Socket.io for Node.js — it a WebSocket wrapper.
You may have noticed I use a lot of Microsoft technologies now. This month I have been diving into SignalR, and it works great. In fact, I prefer using the RPC style instead of Web API because you can serialize almost any parameter, multiple in fact. Anyone who has used Web API knows how your GET request can get funky trying to use the FromURI attributes and don't even try a POST request with multiple parameters. You can find out more from the SignalR slides that I wrote.
Another reason why I have just adopted these technologies, but embraced them is that the tides are turning for the better. Microsoft appears to be open-sourcing a lot of projects, especially Web stuff. If you haven't heard of vNext you should check it out now. It looks promising.