Quora on MySQL failed outright when AWS EBS failed, companies on AWS using Cassandra like SimpleGeo and NetFlix did not. To their credit, Facebook were clear enough on their reasons for using HBase over MySQL and Cassandra, such as wanting to double down on their current Hadoop system/knowledge and having easily obtainable ordering guarantees on messages. It's also clear they've invested in making HBase good enough.

At large loads and footprints, imvho, Riak, Cassandra and HBase present viable options. But there are some factors to consider that don't seem to get mentioned in the pop tech press

- What are you able to operate in production?

- What are you able/willing to debug and patch?

- What hardware options do you have?

- What are your workloads?

- Which variable of C.A.P, when you lose it, most damages your business?

- Will your company's choices be evaluated in the press?

- Does your board/investors have capital tied up in business's that are using something else?

- What architecture tradeoffs and styles sit well with you?

- What kind of data access and consumption patterns make you money?

- Can you pay for help?

The right choice is context sensitive, and I'm fairly sure for this class of systems at this point in time, there's no free lunch. That means you have to do the legwork for yourself and make your own choices and commitments; doing what you heard worked for someone else is a cargo cult.

Or maybe it's wisdom. "A fool learns from his mistakes, but the truly wise learn from the mistakes of others." -- Otto von Bismarck