I'd laugh at them, probably. I had no regular tormentors, but the few people who I can recall being mean to me turned out pretty pathetically, and I'm far better off as a person than they are. What happened then made me who I am today, and it's far enough in the past that it's irrelevant to me today.
I love hearing stories about bullies finding their prey on Facebook now that they're all grown up. One of two things seems to happen - they either apologize for their behavior way back then (which is usually met with shock, silence, and/or disdain), or they have no clue how horrible and scarring their behavior was and act like just the fact that they were classmates/friends/neighbors way back when is enough to warrant being BFFs online (which is usually met with shock, silence, and/or anger). Either way, what's the point? Move on, folks, move on, and leave those you were cruel to be unless and until they contact you first.