What is the Reason large sites don't use MySQL with ASP.NET?

Adding into what AJ said... Remember Facebook also pays C programmers to hack up MySQL code and also PHP code to get things to really work "well" for the amount of traffic they get.

Facebook already made statements in the past and this year about having wished they made a better choice.

As a matter of fact, for coding they're now compiling their PHP down to C++ code using HipHopPHP and about 90% of their servers are running the C++ binaries instead of the PHP scripts.

Their MySQL database might save them a dime or two, but the costs to maintain it, scale it, etc. is extremely intense.

A product like Oracle however would really allow you to scale seamlessly compared to MySQL.

I have a site right now that uses a lot of bandwidth on my database, large number of queries, and the truth is, scaling is a pain in the neck with MySQL and their Clustering product isn't that great and requires a license. Oracle right now has the best "grid" database setup but the costs are insane there...

Also, I code C# as well.. Let me tell you it's MUCH easier to integrate enterprise level sites with SQL Server compared to MySQL.

I would guess that it's probably because it's really really easy to get started making a site with ASP.NET hooked into SQL Server. And for the sites you mentioned, speed to market was probably more important than getting the architecture "right" (not to say that SQL Server is or isn't the right choice - just that speed to market is the priority). Remember that a developer's job is to release software.

So long as one avoids using too many database specific features, it will be relatively straightforward to switch to a different database with moderate effort. But why bother unless your site becomes super-popular?

Edit: And if you become super-popular, you may even want to venture into the land of NoSQL.

While this doesn't directly answer your questions I really have to refute your comment about outrageous licensing costs. ALL ENTERPRISE grade commercial software comes with a high price tag because it has the VALUE for it. If it doesn't have that value, it wouldn't be a successful product.

SQL Server's pricing is extremely competitive and has a very substantially lower TCO than Oracle. Another reason a decision to use MS SQL Server would be made is that most shops that develop on the Microsoft stack are Windows Server shops. MS SQL Server is built specifically for Windows Server so it can integrate as flawless as possible with the operating system. Many other products are not primarily and solely developed for Windows Server so this results in feature differences and environmental bugs.

These enviromental issues can be further compounded with the fact that large scale shops will employ primarily system administrators that have long backgrounds in that specific stack so in a .NET shop most system administrators are all most fluent in Windows Server, having to support multiple operating systems becomes a large cost especially in the risk management side when you're a large scale business.