Messaging apps have exploded in popularity this year, and few are growing faster than WhatsApp. The global messaging phenomenon announced today that it has reached a significant milestone: 400 million active users each month, with 100 million of them joining in the last four months alone. "We’ve arrived at this point without spending a dollar on targeted ads or big marketing campaigns," co-founder Jan Koum crowed in a blog post. "We’re here because of all the people who share their WhatsApp stories with co-workers, friends, and loved ones."

WhatsApp, which makes money by charging users one dollar per year in exchange for unlimited texts, has become a popular global alternative to the overpriced text-message plans sold by carriers. It has continued its rapid ascent despite competition from dozens of similar apps, including Snapchat, Viber, Kik, LINE, MessageMe, Tellit, iMessage, and Facebook Messenger. (Tellingly, almost none of those companies will say how many people use their app each month.) Users of messaging apps can be fickle — it's quite easy to switch to a new one once your current favorite loses its luster. But for now at least, WhatsApp is still rapidly growing.