This database was open-sourced by Facebook in 2008, being used by Digg, Twitter, Reddit, Rackspace, Cloudkick, Cisco, SimpleGeo, Ooyala, OpenX... It has been developed by Apache, making what they call Cassandra DB or Cassandra DataBase one of the most important databases with a very promising future, even despite not wanting to follow the SQL relational model.
With the simple guarantee that several of the most scaled services, that have to support the highest user loads, trust Cassandra, is already enough for this development. The fact that it is managed by the renowned Apache Software Foundation, whose HTTP server competes and beats many other commercial servers, is evidence of its validity.
Its enormous scalability, its decentralized functioning and its tolerance to faults, polish up one of the best databases available at present.