How many programming languages do you use? :-) Just like we use Python, Java, Scala, etc... where each is appropriate, we also use different databases where appropriate.
Each system serves a need. We also use off the shelf databases, where appropriate: it should be noted Sensei uses Lucene (a well known search library), Voldemort has a pluggable storage engine (where we mostly use BerkeleyDB and a custom read-only storage engine), and Espresso uses MySQL.
We don't build databases for reasons of NIH: we focus on building features (faceting, real-time indexing, partitioning, fail over, etc...) that enable us to build fast, usable, feature rich, scalable, and reliable applications. We readily use open source components in many places within both our infrastructure systems (search, various databases, Kafka) and in the applications, and contribute to existing open source projects.
