like you are so cool，wow，sth. like that~

Let me tell you the real meaning. Someone types 666 to you means that you have done something well or you are very skillful at something. He is praising you!

In China, many netizens or young people would use 666 as a compliment when they see some people doing sth so well, like playing games, musical instruments or being good at sth etc. 666 also sounds like 溜溜溜 ，means things go well, smoothly,so it can be a good wish too.

666 is a number associated with the devil.

666 is called the number of the beast, but there is not much explanation available anywhere. 666 is also a song by Iron Maiden.