When's the last time you replied, promptly, to a message sent through Facebook? Aren't sure? Well Facebook, craving more real-time usage of its service, hopes to change that.On Wednesday the company said it was acquiring WhatsApp, a leading mobile messaging service used by people as an alternative to telecommunications carriers like AT&T and Verizon. Facebook said it was paying $16 billion for the five-year-old buzzy upstart.