https://blog.serverdensity.com/does-everyone-hate-mongodb/ https://blog.serverdensity.com/does-everyone-hate-mongodb/ They use it as well for similar purposes.

https://blog.serverdensity.com/does-everyone-hate-mongodb/ https://blog.serverdensity.com/does-everyone-hate-mongodb/ They use it as well for similar purposes. So it isn't a strange choice. That said, I'd use Cassandra as well. ;) I found when testing the write performance was significantly better. However, I didn't test the special Mongo variant they are using.