In the first part of last year, facebook wasn't blocked at wits. People would stay on that thing for ages while those of us who had important stuff to do (like download porn) had to sit and wait for them to finish. Then facebook got blocked and the computer labs were always empty i.e there was always one or two pcs free.
Youtube during working hours and probably other sites. They're pretty anal here though, shaming people who have spent a certain amount of time on particular websites. The company I used to work for didn't block anything...and I never heard of a lot of complaints from IT.