Internet filtering is appropriate when conducted by your employer. At my work, pretty much everything is blocked- forums, social networking, porn, shopping, FB, personal emails, proxys, etc. I'm perfectly OK with it, since they are paying me the $ for me to do a job, not wast time on the Internet. I do plenty of that at home and on Sundays.
I would image the most of schools are blocked for much he same reasons- you are/were there to learn, and the taxpayers are not wasting billions of dollars a year to have you play flash games and browse Faceboook.
However, under VERY few circumstances should any website be blocked to private access. Short of VERY illegal content (Child porn, torrents, planning of illegal activities, etc), blocking websites does amount to censoring free speech. While it may be OK for China to stifle reports about the devastation from the latest earthquake (in China), it is NOT OK to block content on a global scale (see: British Libel laws, for example) as that amounts to censorship of free speech.