I've always thought the same thing, PD - first generation fans are those who watched the show during its original ABC run, and second generation fans are those who were somehow exposed to DS afterward, whether it was through syndication, the '91 series, Sci-Fi, VHS or DVD. I've never really broken it down into third, fourth or beyond based on the various syndication runs and video releases, the revival series, or a fan's actual generational relationship to the original generation that watched. But I it can certainly be broken down in any number of ways that anyone might want to do it.