The act of writing on someones facebook wall to make either yourself or your friend seem more fun/cool/witty/popular. It is often done with the intention of someone in particular reading it.
Q: Why did Sarah write that on Janes facebook wall rather than just text her?
A: She was obviously peacocking
Jane: I really want Max to think that I am moving on!
Sarah: Don't worry, I will peacock you later about last night