Am I overreacting? Should I leave him?

You are over reacting....its just half naked friends that he most likely just knows thought myspace but never in person....its harmless...guys are guys they like to flirt even if they know it ain't going to get them anywhere...its their instinct.....nut he knows your the one for him.....or else he wouldn't even be with you now would he....who would put up with your jealousy if it wasn't because he does care about you and love you.