WhatsApp is one of the fastest growing messaging services on the market and their latest user count backs up that claim. The company announced the addition of 100 million active monthly users in the past four months alone, bringing the total user base to 400 million.

In a blog post on the matter, co-founder Jan Koum highlighted the fact they surpassed the 400 million milestone without spending a single dollar on targeted ads or big marketing campaigns. Instead, they arrived at this point because of the people that have shared their WhatsApp stories with co-workers, friends and loved ones.

The 400 million active users are responsible for 16 billion inbound messages, 32 billion received messages and 500 million photos sent daily. As such, WhatsApp is now the largest cross-platform social media messaging service in the world as it boasts 150 million more monthly users than Twitter.

WhatsApp and similar services like iMessage, Kik, Snapchat and Facebook Messenger are finding success as mobile phone users continue to shy away from expensive carrier-based text messaging plans. Texting plans weren’t always so expensive but in recent years, wireless providers have modified rate plans to leech more money from data and text messaging services to reflect the changing industry landscape.

WhatsApp is currently available for most popular platforms including Android, BlackBerry, iOS, Nokia and Windows Phone. The app is free to download and try for the first year. After that, you’ll need to fork over just $0.99 per year to continue using the service.