In Brazil more than 90% of smartphone users use whatsapp daily. Whatsapp is basically the only way people to communicate via cellphones here. No one has Line or Sms. Whatsapp is also very popular in other countries with more than one billion users. Germans, Indians and South Africans use Whatsapp. I strongly disagree with Barbara who says "it's good for teenagers" or with Jack who says "it's fancy". They just don't know how good Whatsapp is for instant messages. It's fast, free, practical and you can do pretty much everything since calling until having a group.

About learning language Whatsapp is good if you just want to text. I have more than fifteen language partners on Whatsapp and we talk more because they simply open more Whatsapp than Italki. I think it's a good idea if you want to text. If you want to talk, Skype is better. (when I call by whatsapp, however, I hear the background of my voice, which is honestly one of the worst feelings in the world)

As a teacher I don't like using the phone apps for teaching language. I need to share texts and videos with my student as we are talking so I usually use Skype or Hangouts. I only use whatsapp as a backup if my desktop computer is not working for some reason. I think of whatsapp as a brief chat tool and not really so good for real language learning. 

That's my opinion anyway.

Good luck with your learning.

Well, both are good options, but it depends upon ones intension.If his/her intension is to learn something then both can be treated as good medium(but whatsapp lacks in video calling),otherwise you can use those as per your wish:) 

Well, it is the most fancy applications for flirt and making friends, nothing more than that if the language learning is not well organized.