From China Digital Space

"Wall" (墙) sounds the same in Chinese as "strong" (强). The character for "strong" is a component of a number of Chinese words. Netizens often replace the character for "strong" with the character for "wall" to refer to the GFW (Great Firewall). Examples of these words with the switched-out character are: "mighty nation," "strongly condemn" and "rape."

Read Netizens Force Fang Binxing, Father of the GFW, Off of Sina Microblog.

Except for those who are able to tunnel through the wall, Facebook, as well as YouTube and Twitter are inaccessible in China.