I'm of two minds here; legally, I think the girls should either be put in juvenile detention for a day or two (not a long time, just enough to scare them silly) for assault. And their parents should be talked to, and they should have to attend counseling classes - and hopefully they'll not be allowed on the internet for a long, long time.
Then again, from my own personal standpoint...and I know that this is probably wrong...I see it as rather fair that they've having /b/ sicced on them. Afterall, if you're old enough to post a video of you assaulting someone and stripping them naked, humiliating them, on the internet...well, you're going to get some backlash. The fact that they knew it was wrong and thought of it as funny (they did say that they 'owned' him)...they deserve to be taught a lesson. I hope that no one takes it too far and actually physically assaults them, of course. As far as I know (and I could be wrong about this) no one from 4chan has used the information to physically stalk or assault someone...just to send prank pizzas, strippers, etc. to their house. Frankly, after what they did to the little boy, I think they deserve it.
In short; if you're old enough to use the internet like a disgusting, sadistic troll...you're going to get trolled in return. It may not be right, but I just can't bring myself to feel bad for these girls in this situation.