The school board that I work for blocks some entertainment sites, as well as obviously inappropriate things like adult websites. They also used to block social networks. At one point, before they became so predominant, even cloud storage services like dropbox were blocked because they were mistaken for illegal file sharing services.
The basic idea was to block sites and services that were deemed to have no possible educational value, but more recently restrictions have become less strict as educators have been finding ways to bring social networks, blogs, and youtube into the classroom for legitimate purposes. Many teachers create facebook and twitter pages for their classes, as well as youtube channels.
It's sort of like you cellphones used to be banned in schools, but now many schools encourage the use of smartphones as educational tools. Personal technology used to be seen as a distraction in school, but now it's such a vital part of 21st century life that most schools are incorporating it into their programs.