The scriptures say we will be judged by every idle word that proceeds out of our mouth. Why? Because words, like pictures, can stay in the hearer's mind for a long time. They can become a beachhead for the adversary to tempt us. They have consequences. Sometimes what we think are just words, just kidding, are not so benign to the hearer.
When I was a kid, my little brother, who had bright red hair, seemed to have a tendency to get into mischief. We wanted to call him Dennis the Menace. My mother got after us for doing so, because said she, "It can plant in his mind the feeling that he should live up to that role." She didn't want that. We stopped, and I believe it avoided problems that could have come from it.
With facebook (written word) and just joking around, I fear that we don't see the potential for harm that our "idle" words can and may be doing. When we make light of sexual things, even gross sin, it encourages sin and erodes walls we have built up to protect against it.
I plead with my children to be care-ful and thought-ful of what you say and write, particularly when you are "just joking around." What you say can lay mines and traps, temptations, you would never place in your siblings' or others' way intentionally.