Given that these investments (WhatsApp, WeChat, Tango, Viber) aren't acqui-hires, there is still a relatively long term horizon over which these investments will play out.
Perhaps if certain areas are completely blanketed in wi-fi/connectivity these apps will make calling / messaging completely independent of cell phone providers (such as ATT, Sprint, etc) which is a massive industry to displace. But then again these providers might be the only ones providing wi-fi access.