If you're looking at fantasy sports as a way for "geeks" to play a part in the social interactions which revolve around sports, I think baseball is the one to focus on. We're talking about a game that is seemingly designed for stats nerds and which understanding of basic statistics concepts actually gives you a huge competitive edge.
The rise of sabermetrics (defined as "empirical analysis of baseball" by wikipedia) has been pretty amazing given that its just math, and it has gotten to the point where TV/Radio commentators talk about this stuff every game.