redletterdave (2493036) writes "In just two months since Facebook dropped $19 billion to buy WhatsApp, the five-year-old mobile messaging app on Tuesday announced its its active user base has grown to more than half a billion people. This is not the first time that an app has seen a major pop in users after it was acquired by Facebook: When Facebook bought Instagram in April 2012, the service boasted some 30 million users. In one month after the deal, Instagram gained 20 million new users. By July, Instagram grew to 80 million active users. WhatsApp seems to be having a similar growth spurt, gaining roughly 25 million users each month since the Facebook deal was announced."