Whatsapp has 450 million users every month and continues to grow on a path to connect to 1 billio.
Mark Zuckerberh has known WhatsApp founder Jan Koum for a long time. Whatsapp claims nothing will change for its users, "You can continue to use WhatsApp no matter where in the world you are, or what smartphone you’re using. And you can still count on absolutely no ads interrupting your communication."
WhatsApp allows for real-time messages, group chats and photo/video/location sharing without paying for SMS messaging fees. WhatsApp messenger is available for iPhone, Android, BlackBerry, Windows Phone and Nokia. The app does use data and "unlimited data" plans are recommended.