"I'm ashamed to admit, but most of the site is using MySQL. Now that Oracle XE is available, I hope we could port some sections to Oracle. However, database replacements are never easy. So, don't expect it too soon. … Every penny we make goes back into running and developing this site. There is no way we would be able to raise the money required to buy a commercial Oracle license." (OraFAQ Forum)
More and more popular sites are moving to MySQL: Yahoo, Google, Craigslist, Travelocity, etc. This open source database has certainly moved into wide acceptance within mission critical business environments.