WhatsApp now has 400 million active users. Yes, active. Not just registered. And it got this far without going the way of rivals.

WhatsApp is a mobile messaging service that sends free texts, photos, videos, and audio messages, and it just revealed via a blog post that the service has reached more than 400 million monthly active users. That number makes WhatsApp the most popular, cross-platform messaging service for smartphones in the world.

That's also a huge userbase increase, considering WhatsApp confirmed in October that it hit 350 million active users, a spike from 300 million in August. WhatsApp even tweeted in June that it had passed a new daily record, which included 27 billion messages handled in 24 hours. To put all this into perspective, Twitter only has 233 million active users. And it just had a big IPO.

“We bet that if our team of engineers could make messaging fast, simple, and personal, we could charge people directly for the service without having to rely on annoying banner ads, game promotions, or all those other distracting ‘features’ that come with many messaging apps," wrote Jan Koum, CEO of WhatsApp, in a blog post.

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Aside from disparaging rival messenger features, Koum used today's announcement to detail some user stories, such as one about a British woman who runs a charity in Uganda. She told Koum that her team uses WhatsApp to send "daily reports, photos, and videos of the children they’re helping", and that WhatsApp helps her build support for the organisation across the globe.

WhatsApp, technically called WhatsApp Messenger, uses your 3G or Wi-Fi to send and receive messages. Users can switch from SMS to WhatsApp to send and receive things like pictures, audio notes, and video messages, as well. The first year is free, but it costs 99 cents every year after.