Anyone who says that MySQL is not scalable has no idea. Facebook is one of the examples for a large deployment of MySQL:
How big is Facebook’s Internet infrastructure? Facebook VP of Technology Jeff Rothschild provided some details in a panel at the recent MySQL user conference. Rothschild says Facebook is now running 10,000 servers, including 1,800 MySQL servers that are overseen by just two database administrators.
Facebook recently surpassed 500,000,000 users – half a billion!