Though I am a terrible networker, I've recently discovered that I know quite a few people in places that I've never been to. I have a friend recently moved to Chapel Hill, North Carolina and a couple from church on their way to Amman, Jordan. Of course I'll help out! And it transpires that I know at least 4 people in each place.
I suspect it's because the vast majority of my friends tend to be mobile and adventurous. Those who aren't so mobile (but nonetheless adventurous) live in places that other people move to, like SoCal. There are some geographic quirks too. I've only visited Utah once but I know lots of people there because of church. Or professionally - my time in Morocco means that I'm peripherally connected to the subgroup of American Arabic-speaking/Middle East-North Africa specialists.
Ironically the one constituency that I don't know very well is my own tribe of Filipino-Americans. It's true that it's more difficult in DC than in California because of the demographics, but still. There's a Filipino Young Professionals-DC group but so far the activities haven't been too enticing. The annual summer pool party? NO WAY.