have real privacy concerns about Viber. Do you know you agreed to have a copy of your address book stored on their server for ever (it's already happened when you installed the app). it's currently free but if they want to make money out of it then they can do what ever they like with your contacts.
Viber is free, so they must be chewing through venture capital at the moment to support the service. When this runs out then they will have to generate income or shut down. Their privacy policy is very sketchy
chats.viber.com/priva cypolicy.html. The company is also based in Cyprus. It is not an EU member so does not have to comply with EU privacy laws.