When what's in the database is directly related to $$$, like account balances, financial transactions, hard asset tracking, billing, SSNs, payroll, and legal compliance, you use a serious tool like Oracle. Would you trust your paycheck being deposited into your bank account correctly to MongoDB? MySQL? Postgres? Not to say you can't try to do it with those tools, but with Oracle, you can be pretty much guaranteed that even if all your data centers simultaneously sink into the ocean, you can recover your financial data to a consistent state at a known point in time, even if it's from multiple snippets of backups. Oracle's consistency and recovery tools are still second to none in the industry.
If you're tracking tweets, dating profiles, blog posts, or network traffic ... then your data isn't business critical and you could even use flat files if you were a sadist and really wanted to.
Look, I use PostgreSQL, Sqlite, and even BigTable for various repositories, they're all very good tools given a use case, but that's not data that would sink the business if it wasn't 100% completely accurate. Those data in those tools can suffer some data loss and, even though it would make for a particularly unpleasant period of work and meetings, the continuity of the business would not at stake.