VIber, the Voice Over Internet Protocol (VoIP) and instant messaging app for Android, BlackBerry, iOS and Windows Phone has broken the 100 million user barrier. Not bad at all, and pretty amazing considering the service has been around for less than two years. Now Viber is going to Symbian S40 feature phones as well as Bada OS too, which will expand it reach further. Right now, 1 in 7 of my contacts is on Viber.
Viber main rival in this arena is Skype. Vibers main advantage over Skype, even other instant messaging apps is that Viber was designed around the smartphone, while its competition started out on the the PC.
VoIP and instant messaging apps rely on a dedicated user name or your email address. Viber being designed around the mobile phone uses your phone number as you user ID. Instead of having to manually add contacts to Viber, Viber scans your address book and finds all your contacts who are already on Viber.
Viber also has a small footprint. Unlike most VoIP or instant messaging apps that drain your battery life, VIber sits in your memory as a cached process. Meaning it is stored in the RAM and not running. When a push notification comes for a message or a call, Viber starts up quickly. In my experience, there is no discernible difference using Viber leaving turned on or off.
And yes, you can turn it off, in the Android version. Even if you turn if off, it will start up push notification comes for a message or a call, though a fair bit slower.
It is an amazing app. You should give it a try.