Earlier this year messaging platform WhatsApp reached 700 million monthly users, 70 per cent of whom use it on a daily basis. This represents a huge leap in growth for the Facebook-acquired company. There were 500 million users last April, rising to 600 million by August. Until now, this popularity was based on its text and multimedia messaging, however, the new addition of voice calling will make it more competitive with the likes of Skype and Apple's iMessage. Voice calling is now available on WhatsApp for Android mobile devices only. This is not yet on Google Play but can be downloaded directly from the official website.

http://www.whatsapp.com/android/