I have read this article from High Scalability about Stack Overflow and other large websites. Many large high traffic .NET sites such as plentyoffish.com, MySpace and Stack Overflow all use .NET technologies and use SQL Server for their database. In the article it says a source in Stack Overflow said:
As you add more and more database
servers the SQL Server license costs
can be outrageous. So by starting
scale up and gradually going scale out
with non-open source software you can
be in a world of financial hurt.
Why don’t these sites use MySQL instead of SQL Server?
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.