Facebook is acquiring mobile messaging company WhatsApp for about $16 billion, plus $3 billion in stock grants, as the social-networking firm looks to boost the overall time users spend on its properties. The price tag for the deal comprises $4 billion in cash and approximately $12 billion worth of Facebook shares. In addition, the agreement provides for an additional $3 billion in restricted stock units to be granted to WhatsApp’s founders and employees that will vest over four years subsequent to closing.