According to the WhatsApp blog:
Thanks to all of you, half a billion people around the world are now regular, active WhatsApp users. In the last few months, we’ve grown fastest in countries like Brazil, India, Mexico, and Russia, and our users are also sharing more than 700 million photos and 100 million videos every single day. We could go on, but for now, it’s more important that we get back to work – because here at WhatsApp, we’re just getting started.
Now that’s quite impressive, not only for the number of active users but for the amount of the activity as well.