Couple of things I don’t see mentioned in the RedMonk post:
if and how data has been normalized based on each connector availability
According to the post data has been collected between Jan.2011-Mar.2012 and I think that not all connectors have been available since the beginning of the period.
if and how marketing pushes for each connectors have been weighed in
Announcing the Hadoop connector at an event with 2000 attendees or the MongoDB connector at an event with 800 attendeed could definitely influence the results (nb: keep in mind that the largest number is less than 7000, thus 200-500 downloads triggered by such an event have a significant impact)
Redis and VoltDB are mostly OLTP only databases
Original title and link: NoSQL Databases Adoption in Numbers ( ©myNoSQL)