To be fair, you have to have a very high IQ to understand xkcd. The humour is extremely subtle, and without a solid grasp of theoretical physics most of the jokes will go over a typical viewer's head. There's also blackhats nihilistic outlook, which is deftly woven into his characterisation- his personal philosophy draws heavily from early internet forums, for instance. The fans understand this stuff; they have the intellectual capacity to truly appreciate the depths of these jokes, to realise that they're not just funny- they say something deep about LIFE.