At my work, they are blocking a lot (using bluecoat filter). Results: I have squid running in the cloud,I am connected via a ssh tunnel and chrome shortcut has the option --proxy-server=127.0.0.1:xxxx to bypass entirely the entreprise block. Many people have similar work arounds.
At a previous job, there was almost no block, but the intranet was giving a link to a page where we could monitor the time spend on all the websites during the month. I think this kind of monitoring (added to the usual signature of a policy chart) is a good deterrent for all abuses.
Filtering software are smarter and smarter. They are expensive, people complain. And they are an invitation to find work arounds. I think its best to get rid of them.