Yes and no. You could block the Facebook domain, but due to things like alternate domains, aliases and forwarders, you can easily pass data to different domains

Yes and no. You could block the Facebook domain, but due to things like alternate domains, aliases and forwarders, you can easily pass data to different domains and still have your data reach the intended recipient. The target can be made ephemeral just by configuring your mail server account. I don't think China wants to block specific content because that's a lot more complicated an approach than blocking target systems. By making the web more e-mail like, you can pass data through the web, and it can reach the intended target without that target being discovered. You could in essence borrow the Tor method, but for plaintext communications over HTTP. You could use the same approach for web data by decoupling the addresses from the content. Federated services and pre-authorized applications is one way to connect disparate content and networks to each other. Another would be a way to submit data to a network in general, and allow others to retrieve it, for example with some pre-shared key or authorized service. You could make this work across different types of data and networks. For example, I should be able to send an e-mail to some address, and an intended recipient could check their Facebook messages and see the data, even though the mail address was something arbitrary.