We're a YC startup in California (Palo Alto) using .NET. We also use other more hip things, but .NET is the majority of our server side stack. We wanted something mature and solid that banks would use for a transaction system. Something that would let us hire great talent and not compete with all our startup buddies over RoR and node.js rockstars. Something that supported a nice mix of functional and imperative programming techniques. C#/.NET fits the bill pretty nicely.