Madame Dishes, you have hit upon my quandary. How DO I deal with these folks if we're at a mutual friend's house and they start giving me more grief about FB? As is, they've raised such a stink about my best friend and his wife (who apparently had the unfortunate timing of defriending them the same day) that it's become an "us or them" in many cases when guest lists are drawn.
The even sadder thing is that the cut direct between Jack and my friend was over a posting on another web forum that was taken out of context by Jack. It makes me sort of hate the internet, but I work online and only keep track of half of my family through FB.
I think that most people, or I'd like to think that most people, will be judging Jack & Jill for their behavior. They will know Jack & Jill from their antics to be dramatic and juvenile. It wouldn't surprise me, if Jack & Jill continue to act out at parties about nonsense, that people will become wise to that and stop inviting them based purely on how they behave
. Not about taking sides.
If Jack and Jill try to talk directly to you about FB, you can say, "I'd hate for the other guests to be made uncomfortable. Let's stick to pleasant topics." Then, turn away from them and talk to someone else/continue enjoying what you were doing before. Basically, be polite, but do not engage further. Keep most of your attention on the people that are enjoyable.
If they try to talk to others about the online nonsense, I'd ignore it entirely. Again, keeping my attention on the people that are enjoyable.
I can say that if a Jack & Jill type person came up to me at a party and started to complain about being defriended or making a scene over something that happened online, right away I'd keep my distance. Don't care what happened, it's just rude to bring it up at a social gathering.
Depending on how big a scene was made, I'd talk to the host/hostess. My last resort would be "I don't want to be, directly or indirectly, the cause of drama at your gathering. I'm afraid that my presence has made another party noticeably agitated. I've had so much fun, and I can't wait to hang out, again. See you at XYZ!" And then I'd leave.