WhatsApp, the well-known popular mobile messaging service has crossed another major milestone recently. The company CEO announced on Twitter that WhatsApp is now service over 600 million monthly active users.

The CEO was quick to point out that it's active users, that is, people actually using the service everyday, and not just registered users, which would be a bigger number but a lot of those people won't actually be using the service. This makes the milestone all the more impressive and shows once again just how popular WhatsApp is.

Now serving 600,000,000 monthly active users. Yes, active and registered are very different types of numbers...

— jan koum (@jankoum) August 25, 2014