WhatsApp's CEO and founder Jan Koum announced on Twitter Sunday that the social messaging app now has 600 million active users each month.

The announcement follows social networking giant Facebook's decision to buy the app in February for $19 billion, according to CNET. WhatsApp is still in the process of being bought.

"Now serving 600,000,000 monthly active users. Yes, active and registered are very different types of numbers ..." Koum tweeted.

While the deal has received approval from the Federal Trade Commission, Facebook still needs international regulatory approval to complete the acquisition.

Koum's announcement focuses on people who use the app on a regular basis, instead of how many times people have downloaded and installed the title on a smartphone or tablet, CTV News reported.

600 million users sending monthly texts, images, videos and emojis has made WhatsApp the most popular social messaging app in the world, with WeChat in second place with 438 million users, most of which are based in China.

WhatsApp had close to 450 million monthly active users when Facebook announced it was buying the app, according to CNET. The company announced in April that it hit half a billion users and that people were using the service to share 700 million photos and 100 million videos on a daily basis.