After having a very recent experience of being the focus of a few rather puerile, insulting comments from a family member on a social networking site, I thought that this would be a good area of etiquette to explore since it seems we don't typically discuss this here and when we do, many people feel justified in executing retaliatory rudeness.
My choice was to not respond to the comments publicly. I may eventually do so in private but there is no way I'm going to get into a public pissing contest with all the relatives watching and drag myself down to the other person's gutter level. I honestly do not think people are stupid and when someone acts ugly and obnoxious while the other person refuses to play that game, people will rightly conclude who is the real jerk. If both people were to start wallowing in the argument pigpen with each other, they both lose, imo.