How many monthly active users did WhatsApp have in December 2013?

WhatsApp had 400 million monthly active users in December 2013.

That milestone was announced by WhatsApp co-founder Jan Koum in a December 2013 blog post titled “400 Million Stories.” The figure measured people who used the service during a month, not the number of messages sent or the number of registered accounts.

WhatsApp’s growth was accelerating rapidly at the time. In February 2013, it had about 200 million active users; by December, that total had doubled. The company’s simple internet-based messaging model helped it spread internationally without relying on traditional SMS plans.

A common mix-up is confusing this figure with WhatsApp’s later milestones. The company surpassed 450 million monthly users around the February 2014 Facebook acquisition, passed 500 million in April 2014, and reached 1 billion in 2016.