Depends on your environment
I'm IT/IS director for a small school district. While I wouldn't want teachers or students frittering away time playing Angry Birds during school time, Dropbox and Evernote have proven to be very valuable tools for teachers and students; I wouldn't dream of blocking them. For students, we block Facebook during school hours, but open it up after school. We emphasize digital citizenship rather than trying to block every conceivable problem website. Facebook and Twitter are not blocked for teachers at all, but we will discuss appropriate use with a teacher who appears to be spending class time on FB rather than teaching/supervising/mentoring students.
Angry Birds can be a good tool for demonstrating and learning some basic principles of physics.
