WhatsApp growth doesn't seem to be slowing down, with its CEO Jan Koum tweeting that the mobile messaging app has over 600 million monthly active users. The tweet from Koum wasn't just to brag about the growth of WhatsApp, but that its 600 million users are actually active, compared to its competitors.
Services such as Kik and Line only report on their registered numbers, so we don't know how popular they are at any given time. When Facebook acquired WhatsApp back in February for $19 billion, the company was edging on 500 million users. A couple of months later, in April, WhatsApp hit the 500 million mark, so it has only taken the company another four months to take in another 100 million users.
Right now, WhatsApp is free to use for the first year, and just $0.99 per year after. With 600 million monthly active users, the company isn't making much of its half-billion user base, but I'm sure we'll see that change soon when the Facebook acquisition closes later in the year.