I never thought about it that way. That's a pretty astute observation.
So the gov wants to maintain control over Chinese interactions with each other, hence any major site in Chinese will get blocked immediate (as soon as Quora allows for Chinese content and it becomes popular, it will be blocked in favor of a harmonized Zhihu).
- They don't do this for economic protectionism (although I know that the Chinese view this as a benefit of the practice).
- and they are unlikely to lift a block on a social network, since social networks and communication channels are what they are primarily afraid of.
Internet companies that don't enable rebellious interactions between Chinese citizens are perfectly accepted...e.g. LinkedIn. Note also that Chinese social media tech, even very successful ones, have had absolutely no significant penetration outside of their protected market...not Myanmar, not Cambodia, none of China's next door neighbors, nor Taiwan or Hong Kong. There are stuck in their own ecosystem, with those that do get to compete with foreign companies, like Alibaba, doing much better outside of China.