Much like rofl, this is the n00b version. The 'z' means nothing - it is just used 'cuz people think it looks cool.
Often teamed with 'xxx1111eleven1one"
Person #1: "My nipple was pulled of when I was on the receiving end of a nipple cripple"
Person #2: "roflz111eleven1111!"
ROFLZ is similar to ROFL, but with flair. Whereas some would contend the "Z" is useless, others, with intelligence, would contend it accentuates the point in a texting based environment. The others would be right.
1: You beat my high score.