I've always wondered why alternative datastores, like MongoDB, get so much good press when a RDBMS like MySQL or PostgreSQL can be used for storing non-relational data in a way that scales tremendously.
There's a video (I don't have the link handy) where a Facebook engineer says they actually had to make code modifications to MySQL to make it do what they want.
They're using the tool in a way that it wasn't designed to be used. Therefore it's probable that that there's an opportunity for specialized software to fill the role better.
The only article I'm seeing for this issue is this one from 2009, by Bret Taylor: http://bret.appspot.com/entry/how-friendfeed-uses-mysql
Other than that, crickets, which is a shame as these RDBMS databases are proven to be more reliable than the fad du-jour.