They don't need to. They just need to stop it becoming a full competitor. Facebook's asset is the social network, your contacts. WhatsApp was a threat because

They don't need to. They just need to stop it becoming a full competitor. Facebook's asset is the social network, your contacts. WhatsApp was a threat because they were getting a social network to match. That could then have been acquired (by Google) or developed a more social platform on top of it. By owning it Facebook can keep WhatsApp limited to a messaging platform only and remove the risk and limit the damage to Facebook itself. Next step moves to encourage WhatsApp users onto Facebook for group messaging type services.