Good questions, David. I've wondered about this for a long time while watching my own personal case study.
17 years ago I launched Adventure Rider, a forum for people who like to ride the Land Rovers of motorcycles. It was obvious from the beginning, at least to me, that it was temporary until a shiny new thing came along. Fine by me, it would be fun while it lasted. It was on forum software that looked dated during an invasion of tech companies with huge teams and millions: Friendster, Friendfeed, Orkut, Buzz, Wave, Yahoo Groups, MySpace, Facebook, Reddit, Instagram, Google+...
My intuition was reinforced after a few years when some of our earliest members drifted away, saying it had become too big and impersonal. Most of them are still gone and we miss them.
But the strangest of things happened: it kept growing to 350,000 members and 2 million unique visitors a month and I feel the general sense of loyalty and happiness is higher now than it was.
Here's my theory: when sites like Reddit get huge, people retire to their corners, but those corners get more interesting because if the network draws millions, the chances of a few people being interested in special interests you have is higher. And you can get a bigger audience for your particular interests. No?