It looks like Twitter has scaled back its move to Cassandra. This article contains an excellent chart matching high level use cases to the database engines that best satisfy them, together with the strengths and weakness they bring. Good stuff for off-the-cuff analysis of proposals that tout NoSQL as the solution to the problem.
Twitter has scaled back its plans to store billions of tweets using Cassandra, a non-relational database project that Facebook created and open sourced. Friday night, Twitter said that it will still use Cassandra in a new real-time analytics project it is building, but the decision to move away from plans to migrate tweets from its current MySQL database to Cassandra is seen by some as a blow to startups and open-source projects that are attempting to move beyond relational databases.