There are many messaging clients and the cool thing at present is for the different clients is to state now many users they have. Except having people signed up is different to actually using a service.
So WhatsApp got fed up with other messaging clients saying they had millions and millions of users and has decided to lay down the gauntlet by stating they have 400m active users.
WhatsApp has a very good business model. Cross platform and free for the first year and then just £1 a year.