Want to post a pic of the Statue of David? Too much nudity. The Vietnamese girl running away from napalm? Same.
Twitter and GMail ar a bit better, but the worry is there. Some Chinese people were worried because their phone disconnected after they said particular keywords. Coincidence, or actual real time monitoring? On Facebook we can see that they are actively monitoring and censoring us, but we keep using them!
Imagine of things like the pipeline protest get classified as "terror events" and Trump-supporting Zuckerberg (Facebook hosted a party with right wing website The Daily Caller, who used Facebook Live Video to spread Trump propaganda to great success, guess who wanted to celebrate that) agrees that talking about it amounts to "support of terrorism". The current occupying regime can even provide fake evidence to suggest "terrorist level" aggression, and if the people on the ground want to refute it, hah, which communication medium do they want to use that would allow that attempt?