Kakao Talk's recommend friends list is simply made up of people who are contacts on your phone, it doesn't use social data such as contacts from Facebook or Twitter - so it's pretty straight forward.
Recommended friends on Kakao Chat
I believe that if someone hasn't got Kakao Chat installed, but is on your phone's contact list - then you can send an invite to them, but you can't add them and starting chatting until they have the app too.
I'm kind of new to Kakao Chat, and only really downloaded it to keep up with former colleagues who now live in Korea, but I have to say the one thing I really like about the app, and that makes it stand out from all the other messenger apps like Whatsapp and Viber is the ringtone!
It's so cute I could listen to it for ages :) Have a listen yourself:
