WhatsApp, the wunderkind startup snagged by Facebook in a $19 billion deal, has reached half a billion active users and doesn't seem to be slowing down.

CEO Jan Koum announced the 500 million user milestone in a brief blog post, saying that the messaging service had grown the fastest in Brazil, India, Mexico and Russia. WhatsApp users are sending more than 700 million photos and 100 million videos daily, Koum wrote.

"On one hand, we were kind of expecting it," Koum said of reaching the half-billion mark to TIME. "We got to 200 million users, 300 million users, 400 million users. It was going to happen sooner or later. But we think it's an exciting number to share with the world and a good milestone to acknowledge what's all been organic growth."

Facebook's purchase of WhatsApp made users somewhat uneasy and inspired the Federal Trade Commission to issue a stern warning to the social network. In the past, Koum has stated that messages are deleted after they go through the systems, leaving no data to be mined. The operation has also stayed tight with just 60 employees to date, despite passing 500 million users.

It's still unclear why Facebook bought WhatsApp in the biggest-ever startup purchase. Mark Zuckerberg, co-founder and CEO of Facebook, could be planning to use WhatsApp to help further Internet.org, an initiative to bring wireless connectivity to undeveloped areas.

For now, it's certain that WhatsApp is rapidly growing. In the TIME interview, Koum detailed how people are using the app, which he plans to make a replacement for SMS.

WhatsApp is primarily so loved ones can "keep in touch with each other, not movie stars or sports stars or random people you meet on the Internet," Koum told TIME. "That's why we're succeeding internationally. We want to do one thing and do it really well. For us, that's communications between people who are friends and relatives."

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