I tested Viber all day. It has a lot of impressive features that I just don't need, including lists of friends, enemies and frenemies from my addressbook. No way to rid some of these people from the user list even if you block them. Deleting a contact on Viber also mean deleting a contact on your phone.
I tried Snapchat, too, but this app is just too much for what I need.
I think Kik does IM best, including a sensible addressbook matching, few friends and even less of the losers.