Here is a source from 2015: http://www.marketwired.com/press-release/metamarkets-clients...
You can also find additional information that folks have been willing to publicly share on scale and use cases here: http://druid.io/druid-powered.html
You can also find additional information that folks have been willing to publicly share on scale and use cases here: http://druid.io/druid-powered.html