Blocking Access to Social Networking Sites

For several years now, many business and school network administrators block access to Facebook, Myspace and other social networking site on the grounds that they distract employees from more important work. Preventing access to Web sites by name is straightforward, but resourceful employees can find a site's IP addresses and enter those into the browser easily enough. Consequently, administrators often try for more sophisticated IP address blocks. These sometimes also be circumvented via proxies. Have you been on networks that block access to social networking sites and tried to get around it?
→ See also - What Is a MySpace Proxy?
→ See also - What Is Facebook's IP Address?