According to the latest news from the below source, WhatsApp now serve 600 million active users.
Earlier this year in February Facebook bought WhatsApp for $19 Billions. Before the acquisition Facebook was having around 1Billion active users and now after acquiring WhatsApp the digits are dipped to 800 Million. But now the thing is, that WhatsApp has reached a new mark of 600 million active users.
Source: @Jan Koum