If you remember in 2010 China hacked into Google's infrastructure and gained access to the interface and databases set up for US intelligence surveillance.

China can and does filter content at a packet level and TLS 'strong encryption' doesn't protect you when CAs are both owned by the government, proprietary Chinese drivers run on Operating Systems, the large companies provide hardware backdoor access to the government (US bans Lenovo for government use for this reason), and networking companies likewise provide all hands access.

Just like any other internet service, China can filter FB/Google content directly - though it is easier with partnership and alas FB/Google has allied themselves with the US.