MongoDB -- as our core application data store. It's popular to make fun of Mongo these days, but we've found it to be a great database technology as long as you don't store too many things in it. Anything you can count on your fingers and toes should be fine.
I suspect this is a joke, but I'm still confused? If 4B things happen every day, how is that finger-toe countable? Does this mean that relatively slowly-changing things like "customer name" are stored in mongo while events etc. are stored elsewhere?