It only seems like it isn't when you read sites like Hacker News, where most of the posters are not working in big environments. Facebook is a large scale solution that doesn't use either, but their data management and caching is so bad nobody should be considering them as a best practice.
It's not about being big, banks and insurance companies simply have different requirements on how to access and store their data compared to most websites/services.
http://www.google.com/intl/en/jobs/uslocations/mountain-view...
Google does use MySQL for their AdSense/AdWords transactional system (ie, buying AdWords).
If you took all of the votes from the 2000 election in the United States (I'll save you the search - it was just over 100 million), it wouldn't even equal one day's ATM transactions in the U.S., and those transactions are a nonevent.
Twitter in not only big in terms of traffic. 200 million tweets per day is not small, even if a tweet involves less processing than an ATM transaction. You can also consider Facebook as an example of website dealing with big data.
- Oracle and SQL Server: chosen by us
- Sybase and Informix: chosen implicitly by choosing applications that preferred them[1]
- DB2: Chosen by a company that we acquired
[1] Yes they ran on Oracle too - but when 98% of a vendor's customers are on Informix or whatever, sometimes it's just less painful to go with the flow, so you have access to that community.