LOLZ?

Laugh out loud zilch

I believe here, when you add this "z", "LOL" become a noun, making the meaning of "LOLZ" "laughs". Or it's just idiots who don't know how to use "LOLZ" or "LMAO" and use "LOLZ" in place of "LMAO". Look at context to tell which one it is.