WhatsApp, the cross platform mobile messaging service, announced today that it now has 400 million active users.

This is not registered users, but the number of active users per month.

In today's blog post from WhatsApp it also noted huge growth in recent months. 100 million active users have been added in the last four months alone!

Founded in 2009 by two engineers from Yahoo, WhatsApp has proved to be a huge success. The App takes the almost unique approach of offering a one year free service, with no adverts or missing features. Each year thereafter users pay $0.99.

WhatsApp rarely release stats. In October 2011 they informed us they were sending 1 billion messages per day so this number must have increased quite significantly since then!

WhatsApp is available on iPhone, Android, Windows, Blackberry and some Nokia mobiles. It is not avialable for tablet devices, or desktop computers.

Despite many requests for Tablet support WhatsApp have so far stuck to the same policy of 'no planned support for tablets'.

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