Honors Society?

I got an invitation to join some Honor Society (which I usually don't take as such an honor, because I really think they are just money-making organizations). But this one came with the elusive distinction of being "the nation's oldest, largest, and most selective" honor society. I thought about that for a moment, and started to wonder - how could they be both the largest AND the most selective? Not sure I want to be a part of an Honors Society who thinks those are not mutually exclusive...