The reddit bot /u/xkcd_transcriber keeps track of xkcd references on reddit as a side-effect, and publishes statistics here.
As I tend to look for Zipf’s law in anything involving ranked frequencies, I plotted a log-log graph of rank versus number of references:
(The blue line is a power law regression, i.e. what the data would lie on if it followed Zipf’s law perfectly.)
Not bad, not bad at all.