By Ryan Bushey From Business Insider

WhatsApp is an instant messaging service based in Santa Clara, Calif., that just reached 400 million monthly users. According to The Telegraph, WhatsApp sends 20 billion messages per day — double the size of Facebook’s daily messaging traffic.

The app was created in 2009 by former Yahoo! employees Brian Acton and Jan Koum. WhatsApp allows you to send text messages, audio, and video to others — as long as you subscribe. But as long as you have a strong internet connection, the app will never add additional roaming charges to your phone.

Apps like WeChat and LINE pose a big threat to WhatsApp since they have the same features and are acquiring similarly massive userbases overseas.

WhatsApp is available for iPhone, Android, Blackberry, Nokia and Windows. Are you interested in WhatsApp?

PHOTO: WhatsApp CEO Jan Koum

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How to use WhatsApp, see attachment.