On the surface, it does seem like social anxiety disorder and extroversion shouldn't both exist in the same person. On the other hand, anongoodnurse has already touched upon a point which leads us to a different conclusion - these traits are on different dimensions. When one takes a closer look at what the two dimensions actually mean, (especially as confirmed in latest psychological research) it seems like they can co-exist just fine, and most people probably have one or two (or many more) aquaitances with both traits (to some degree or other). anongoodnurse has already described SAD. Unlike it is traditionally assumed by the layman, the introversion-extroversion scale does not measure a social property, however. Rather, it describes to what extent a person derives rewards from stimuli - extraverts simply derive more pleasure from environmental stimuli (of any kind).  Recent neuroscientific research has confirmed this: modifying the neurotransmitter systems of reward/punishment (i.e. dopamine) modifies introversion/extraversion.  Of course, an arbitratily chosen person who derives pleasure from the environment probably also usually derives pleasure from social environments. However, this is surely not always the case - there are plenty of people in academia, for instance, who love exploring environments - just not social ones ;).
 Gray, Jeffrey A. "The psychophysiological basis of introversion-extraversion." Behaviour research and therapy 8.3 (1970): 249-266.
 Depue, Richard A., and Yu Fu. "On the nature of extraversion: variation in conditioned contextual activation of dopamine-facilitated affective, cognitive, and motor processes." Frontiers in human neuroscience 7 (2013).