Facebook has bought smartphone messaging app WhatsApp in a deal worth a total of $19 billion in cash and shares. It is the social networking giant's largest acquisition to date. WhatsApp has more than 450 million monthly users, and the company claims it is signing up about 1 million new users a day. In a press release announcing the deal, Facebook founder Mark Zuckerberg described WhatsApp's services as "incredibly valuable". Analysts say that Facebook made the acquisition because Whatsapp appealed to younger users, and because WhatsApp actually had greater penetration in some international markets than Facebook. Some industry experts also said that the price Facebook paid was not reflective of Whatsapp's true value.