See, OK is a multi-purpose word that can be used as an adjective, adverb, interjection, noun and verb. The meaning that is used the most is "Yes / Surely / Fine / I see". In Chat language, it has been shortened to 'K'. This is why you see K being increasingly used nowadays in chats and SMSs. When you want to say an Elongated K (In excitement, or in complete agreement), people tend to write KKKKKKK. So, there is the answer to your question :)

It's just like 'lol' from american people used to... So it's kind a of smiling, means that you're laughing about something =)

I was talking to someone from Brazil and they said it is equal to our LOL.

I have been seeing RSRSRS quite a bit as well :O)

You found very interisting things, I'm interested in your questions! And they help people who would like to know them, good job, Mario!