I’ll let Xi Jinping answer that for me:
Xinhua Xi said, “the development of the Internet has posed new challenges to national sovereignty, security and development interests.”
Facebook et al. are foreign companies. China prefers local companies over foreign firms and often puts restrictions on foreign firms to protect homegrown talent.
For example, there is a limit on the number of Hollywood movies that can be shown per year, so only the biggest names come to China and the domestic movie industry and grow. Same goes for the internet. China wants its domestic firms to develop and not get shouted out by multi-billion dollar Giants.
China likes to control the message and censor the media, some foreign companies play ball (Yahoo! and Microsoft) while others refuse (Google, Twitter, Wikipedia). Google used to comply with Chinese censorship, then suddenly it withdrew and redirected all its search traffic to their Hong Kong site instead of the .cn site.
Another point to make is that Facebook et al. might not care that they are blocked. Google still does business in China. They have offices in Beijing and they sell advertising to Chinese firms. Likewise Facebook’s CEO, Mark Zuckerberg is no stranger to China, he has even learned the Chinese language to an acceptably high level. He has discussed/made major deals with Chinese internet firms.
There are two ways to enter a foreign market. You can take your own brand into the country and try to eek out a bit of the market share over many years, or you can just buy into one of the existing players in that country and get instant market share. Even when Google was in China, it didn’t have the success that has had elsewhere. Yahoo has been in China for a very long time but has always struggled to capture the market even back when Yahoo was bigger than Google (remember then). Facebook also was never really popular in China. Facebook may have more success in the Chinese market by taking a stake in Chinese social media firms in just the same way that Yahoo has a huge stake in Alibaba.
Wikipedia? but that isn’t blocked is it. Well, it is partly blocked. The Chinese version of Wikipedia is blocked and sensitive pages on the English site are blocked. Before 2008, Wikipedia was completely blocked.