It was announced today that Facebook would buy the Whatsapp company for $16 Billion in stock and cash. As part of the acquisition, Whatsapp founder, Jan Koum, will join the Facebook board of directors to help shape the future of Facebook. Though it was noted that Whatsapp will maintain independence and operate separately from Facebook much the same way as how Instagram operates. So it doesn't look like Whatsapp will be replacing Facebook Messenger anytime soon.
This move gives Facebook a foot in the door in this quickly growing segment just as it's user base starts to contract. Whatsapp has 450 million monthly active users users, around half the numbers of users Facebook reports.