Google Talk, Facebook messenger, ... then it is not even dependent on the mobile phone number. And I like being able to type the messages if I have a keyboard and not just my mobile phone. I cannot understand why everybody uses WhatsApp.
It depends on what you want to do. If you want to make sure your little brother isn't spying on what you're saying, any of the IM platforms from established players is likely "good enough" (gtalk, skype, facebook chat, etc). If you're a dissident in the middle-east, your requirements may be difficult to meet.
Like WhatsApp, Viber is free to use and has no advertising model. If they are not making money off me directly i have to wonder how safe my data actually is with this service.
WhatsApp is not free to use. iOS users are charged a flat 99c fee while Android users are charged 1$ a year from the second year onwards.
I do not know how secure Viber is but they have been steadilu acquiring good user base. If I was Viber, I would cash on this opportunity to write a blog or advertise their security models.
Textie Messaging on iOS and Android is all SSL and doesn't scrape your address book. It can also text with people that don't have Textie by sending free SMS or email.
