Fair enough. I think that the community aspect is a competing theory here - or even a complementary one. I've personally (face-to-face) dealt with conspiracy believers that tend to be isolated in their beliefs, but I totally buy that for many, it's the shared belief that matters, almost regardless of what the belief is even about. This also has support of some sociological research I remember reading.

About the Doctorow's idea, I don't know. Do we have increased amount of conspiracies? Or perhaps just a perception of it? Or maybe we're constantly exposed to micro-conspiracies - namely all the businesses, big and small, scheming how to one up each other and screw up their customers - that make people prone to see conspiracies everywhere?