WhatsApp Messenger is a proprietary, cross-platform, encrypted instant messaging client for smartphones. It uses the Internet to send text messages, documents, PDF files, GIF images, video, user location and audio messages to other users using standard cellular mobile numbers. As of February 2016, WhatsApp had a user base of one billion, making it the most popular messaging application. WhatsApp Inc., based in Mountain View, California, was acquired by Facebook Inc. on February 19, 2014, for approximately US$19.3 billion.