In the old days (even 20-30 years ago), geography served that purpose, and after that, what seems to have been a golden age where vaguely like-minded people could congregate and get to know each other on amateur radio, CB radio, BBSs, usenet, IRC, etc.
It seems like these days, all we have are massive communities like facebook, meetup, etc, and although there are interest-specific groups, there just isn't any friction to keep the idiots out, and you're not much more likely to meet interesting people there than you are on the street or in a subway car.
I'd be interested to hear of anybody's experience with any modern communities that have even just a tiny barrier to entry, which serves the purpose of making them even just a little bit more efficient at allowing people to find each other and network.