It's an endless discussion. Just Turkcell's databases bigger than all of these firms count:
http://www.oracle.com/us/corporate/customers/turkcell-exadata-snapshot-189469.pdf

I never denied for a second on whether Oracle has very large installations. For a long time, Oracle was a better choice for larger environments.

However, functional enhancements in SQL 2005 and major architectural enhancements in SQL 2008 have significantly changed the way SQL Server is now perceived. Some organizations are pushing Oracle out the door because they can get the functionality and performance they need from SQL Server, at a lower cost.

Once upon a time, the marketplace spoke - that SQL Server was often not a great choice beyond small and medium sized companies. The marketplace is speaking again - that SQL Server is now being accepted as a viable choice in areas where Oracle and other vendors previously held an advantage.

The public sector and health care were two areas in particular where Oracle once enjoyed much higher clout than SQL Server. That is changing. Oracle is certainly not going away....but SQL Server has been catching up.