On the eve of the anniversary of the 1989 Tiananmen Square massacre, China has cracked down on online discussion of the event by blocking Twitter and a number of other Internet services. Users based around China have begun reporting that they can't access the popular 140-character liveblogging service as chatter about Tiananmen Square has ramped up in recent days, nor can they get onto photo sharing service Flickr, Hotmail, or Live.com/Bing.com.

A brief history lesson for those who aren't familiar: in 1989, demonstrators gathered in Beijing's Tiananmen Square to protest China's authoritarian government. The group was led mostly by students, but over a period of days, it's estimated that several million people joined the students in the Square. The government was irked by this "social chaos" and tried to convince protesters to leave, but did not succeed. Long story short, tanks eventually rolled through Tiananmen Square and fired on unarmed protesters. China has never released an official death count, but estimates range from hundreds to up to 7,000.

This topic is one of several that China aggressively targets when censoring the Internet in recent years, and with every anniversary of the incident more social networking services are available online for people to converse about these forbidden topics. This has forced the government to stay on its toes and add more and more sites to its Great Firewall list. WebSitePulse's Great Firewall test fails to make a connection to Twitter, Flickr, and Hotmail from Beijing or Guangzhou, though reports across the Web show that some users can get spotty access in different parts of China.

"The whole Twitter community in China has been exploding with it," technology "commentator" Kaiser Kuo told Reuters in reference to chatter about the Tiananmen Square incident. "[The block is] just part of life here. If anything surprises me, it's that it took them so long."

We have learned in the past, however, that the blacklist is always changing—sites that are blocked today may not be blocked next week or next month. For example, China has on and off blocked access to Wikipedia, or sometimes just certain Wikipedia pages, for years now. The same applies to Blogspot, Xanga, and a number of other blog hosting sites. Once the Tiananmen Square anniversary passes, various parts of China may well restore access to Twitter, Flickr, and Microsoft's services, but for now, social media addicts will just have to do without.