In the land of the instant messaging applications the Monthly Active Users (MAU) is king and right now it seems that WhatsApp is king of the hill. Yesterday WhatsApp CEO Jan Koum spoke to AllThingsD about the messaging app’s latest MAU numbers and said that they had now reached over 400 million. That means that there are now over 400 million people in the world who send or receive a message on WhatsApp at least once every 30 days.

WhatsApp breached the 300 million user mark in August and added another 50 million gain by October putting them on a roughly 25 million user a month gain. Putting than number into context, BBM only has 80 million users.

Koum said that users are now sending around 16 billion messages a day and receiving around 32 billion messages a day. He added that users are now sending around 500 million images a day to each other as well which dwarfs the likes of Instagram (55 million a day), Facebook (350 million a day) and even the hottest photo sharing social network these days Snapchat (400 million a day).

WhatsApp charges $1 a year to use its service but only begins charging from the second year of use so they are potentially sitting on a $400 million a year revenue stream if all of these users continue to use the service for the foreseeable future.