Just because a database platform is good enough for Facebook or Google, that doesn't mean it's good enough for online banking or e-commerce. I believe that financial and operational data, where every transaction has a dollar amount attached to it, is held to a higher degree of scrutiny than a page full of web search results or some teenagers guest book.

NoSQL databases may be the answer for a write one / read many datamart serving a content management application, where all you are doing is bulk loading and bulk reading, and the end result isn't audited by accountants. I'd bet that even Google and Facebook are running their own internal CRM and financial database applications on SQL Server or Oracle.