Brian Acton and Jan Koum founded WhatsApp in 2009 as a cross-platform messaging app available on iPhone, BlackBerry, Windows, Android, and Nokia. It allows users to send messages, photos, and videos without incurring international charges. WhatsApp gained popularity quickly, reaching over 200 million active users by 2013 and 700 million by 2015. In 2014, Facebook acquired WhatsApp for $19 billion.