ASP.NET could mean a bunch of programming languages and I'm assuming that a ASP.NET Server doesn't disclose that.
It's probably safe to guess mostly C#, but that requires a different metric.
'Tis true. I used to know an absolute ton of big sites that were written in 100% VB.NET ASP.NET. I'd still be writing all my backend in VB.NET if it wasn't a second tier .Net language these days.