I use whatsapp which, I'll readily admit, is not without its flaws, but it's available across multiple platforms and has a fairly large user base. It also runs on blackberry in addition to iOS and android and, fwiw, symbian. I also like the fact that it registers you and your favorites by phone number - people have many email addresses but typically only one mobile number. I've found so many people I likely would never have connected with otherwise, show up in my favorites.

I've never tried Kik, but I know several people who signed up when it first came out, but then stopped using it after a while because they realized there weren't that many other people using it.