In blocking access to a large number of websites ahead of Thursday’s 20th anniversary of suppression of Tiananmen Square protests, officials within the Chinese government are trying to strike a unique but pragmatic balance.

They probably are going farther then ever before, cutting off citizens from sites as varied as Yahoo’s photo service Flickr.com, Microsoft’s Hotmail e-mail system and current Silicon Valley mascot Twitter, which rapidly broadcasts 140-character statements to internet subscribers. Yet they appear to be leaving social networking sites MySpace and Facebook alone.