Facebook has entered into an agreement to buy WhatsApp, the popular messaging app for $16 billion in cash and stock. WhatsApp is a global messaging platform with 450 million MAUs, for approximately $19 billion. It’s one of the biggest tech acquisitions since HP bought Compaq for $25 billion in 2001. It means that WhatsApp, which raised a comparatively measly $8 million since its 2011 launch, is now worth nearly $20 billion. The deal is by far Facebook’s largest acquisition to date and comes after rumors in late 2012 and early 2013 that Facebook and Google were vying to buy the messaging service. According to the filing, Facebook has agreed to pay $12 billion in stock and $4 billion in cash for the company. Facebook has also agreed to pay an additional $3 billion in restricted stock units to WhatsApp’s founders and employees as part of the deal, which would bring the total deal price to about $19 billion. WhatsApp, founded in 2009 by former Yahoo engineers Jan Koum and Brian Acton, is the most popular of a set of mobile messaging apps that some argued posed a threat to Facebook’s business. The app served as an alternative to SMS by letting users send messages for free across smartphone platforms. It currently has more than 450 million people who use the app every day.