If competition with Chinese companies were the reason, Amazon, Apple, Bing and so on wouldn't be available in China either.
The difference between those and and the sites that are blocked (not just Facebook and Google, but also international newspapers) is that they cooperate with the Chinese government by censoring their websites, and the others do not.
Economically, Chinese social media isn't very relevant to the government. Since they are not exporting anything, protectionism in that sector doesn't really pay off. But being able to control what the populace can see and hear about the world is very important to them.
So the major reason for the internet blocks is security. It's the security of the state against its own citizens, but security nonetheless.
The difference between those and and the sites that are blocked (not just Facebook and Google, but also international newspapers) is that they cooperate with the Chinese government by censoring their websites, and the others do not.
Economically, Chinese social media isn't very relevant to the government. Since they are not exporting anything, protectionism in that sector doesn't really pay off. But being able to control what the populace can see and hear about the world is very important to them.
So the major reason for the internet blocks is security. It's the security of the state against its own citizens, but security nonetheless.