Hiya. I wrote the article (for verification see Twitter handle @mappingbabel). In poll of 20 $1bn> startups vast majority used MySQL, though I saw a few mentions of Cassandra and MongoDB. Additionally (though not really DBMS) tons of ElasticSearch and Redis. We pull out Cassandra DB specifically because we were able to find a reasonable price comparison example for it (most people I spoke to were just rolling their own MySQL rather than going to a vendor).