Zizek seems introverted to me.
I thought he would be an extravert as well until I read much more about his personal life and panic attacks. From my understanding, there is a verifiable neurological difference in brain chemistry between introverts and extraverts. Introverts process information through Broca's region and are hypersensitive to dopamine, as well as not converting adrenaline to dopamine. Extraverts process information directly bypassing the frontal lobes, and in fact require adrenaline to generate dopamine. Thus extraverts enjoy eye contact but it freaks introverts out -- extraverts enjoy observing but introverts like to talk and reflect. (Zizek has the hardest time observing -- he can't turn off his constant introspection).
If we think of dopamine as something akin to "narcissistic supply" -- something which gives our lives meaning, as opposed to merely happiness -- then I think it is plain to see that Zizek is an introvert. He does not require adrenaline, in fact he abhors it. Zizek suffers from crippling panic attacks, shyness, awkwardness and so on. He absolutely does not require adrenaline to feel good about himself. Introverts feel an adrenaline rush when the phone rings. We typically have a hard time making eye contact, have a lot of nervous tics etc. We also hate the sound of our own voice or watching ourselves on video. (Zizek notoriously avoids seeing himself speak).
That being said, my initial typing of him was ENFJ before I read about his personal life. Discovering that he has panic attacks and is deathly afraid of people (even going so far as to pretend he is in meetings to avoid having to meet with students) seems quite introverted to me.
He is definitely not an INFJ because they are averse to antagonism. INFJ prefers to keep the peace and make connections, and is ultimately on the side of mercy rather than justice. INTJ is very justice-oriented, and when Zizek says things like "I align myself with Jesus with the fiery sword who hates his parents" and so on, I can't help but think INTJ.
The other reason I would think Zizek is an INTJ is because we are often confused with ENTP, and that is one of the common typings for him. I myself have been confused with ENTP on numerous occasions. (We do share similar functions and weaknesses).
INTJ has extraverted thinking which soaks up anecdotes and stories. I know that when I write, it is chock full of jokes and little tidbits taken from my near-photographic memory of quotes and books I've read. I am disconnected from the present (which is my inferior function) and even more disconnected from the past (Si is the 8th function in the stack, the shadow of my inferior). I recognize these qualities in Zizek too: the disembodiment, the over-excited mind which is not present in the moment nor dwelling the past, but always in the realm of future possibilities -- always travelling forward and backward in time to draw conclusions and build systems.
My vote for Zizek is INTJ simply because it's the most similar to ENTP of the introverts. Since most people classify him as ENTP, and since he is definitely not an extravert, I think INTJ fits.