We did AMA and aren't very social-- we like to be polite and it's nice to have the occasional conversation to find out where people are from, but for better or worse we are on the cruise to spend time with people we know, not try to make new friends.
Not sure if our cruise was particularly undersold but we had no problems eating by ourselves-- we generally took a booth which seats 4 and no one ever joined us. And yes, on excursions and in the lounge it was no issue to smile and nod and people and not really engage if you didn't feel like it.