Like clockwork, WhatsApp announced another milestone Thursday, claiming 400 million monthly active users.

In April, the company said 200 million people were using its smartphone-messaging app. Every two months since, it has said another 50 million people were on board.

WhatsApp allows users to send text messages free over the Internet, bypassing wireless carriers that may charge users to send messages over their networks. Just as stable as the user growth has been actual engagement: Today, users send an average of roughly 40 messages each per day on WhatsApp — the same number they were sending back in June.

In an interview, WhatsApp Chief Executive Jan Koum attributes the steady growth to his company’s “focus on messaging.” Unlike competing messaging apps that make money with advertising or games, “we want to get out of the way. We want to let people have a conversation.”

WhatsApp generates revenue through what he calls its “freemium” business model: the app is free for the first year but costs 99 cents a year thereafter. That’s not much, but the company has few expenses with only 50 employees and is profitable, according to a spokeswoman. Neither Koum nor the spokeswoman would discuss financial details.

WhatsApp has “no plans to sell, IPO, exit, [get new] funding,” Koum said.

“Despite the fact that we’re able to monetize today, we’re not focused on monetization,” Koum said. “We view monetization as five, 10 years down the road. We’re trying to build a sustainable company that’s here for the next 100 years.”

Koum said WhatsApp has attracted new users without spending on marketing.

It is a thread-the-needle approach: Koum doesn’t want to risk driving away users with advertising, but he wants to make enough money to keep the business going. “That will allow our company to stay independent.”