Facebook has bought popular messaging service WhatsApp in a landmark deal worth $19 billion in total. The transaction involves $4 billion in cash, $12 billion in stock and $3 billion in restricted stock that vests over several years. The deal is Facebook’s biggest to date and is worth more than the company raised in its own IPO. WhatsApp was founded five years ago as an alternative to SMS, rocketing to 450 million active users in that time and adding another million daily.