Based on their use case, I'd expect they went with either HBase or Cassandra. I'm quite partial to HBase, it's insanely scalable and has a lot of pretty amazing

Based on their use case, I'd expect they went with either HBase or Cassandra. I'm quite partial to HBase, it's insanely scalable and has a lot of pretty amazing features, but at the cost of knowing exactly what you want to do with your application beforehand. I'm not too familiar with Cassandra, but the scalability of an HBase table is almost entirely dependent on your key design. Judging from their use case and requirements, they would likely use a incremental key design which would allow for super fast range scans, of course, this leads to region server hotspotting, which may or not may not be a big deal to them.