I agree Facebook totally compromises user privacy to begin with but that doesn't change the concept of censorship. You're quite right that there's very little to no privacy on Facebook which is why I find it incredibly stupid for anyone to be running a child porn trading ring there. But if Facebook is going through people's secret groups and monitoring the content of them and moderating them then that's still an invasion of privacy.
How does it invade privacy to take photos of a server? Well let me ask you, if you got yourself a storage locker somewhere and stored a photo album in it would you consider it an invasion of privacy for someone to open up your locker and start taking pictures out of your album they didn't like? The data doesn't belong to Facebook but rather to the users of Facebook. Granted it could be argued that users agree to this invasion of privacy when they use Facebook but it's STILL an invasion of privacy. I'm not arguing that Facebook is somehow known for respecting privacy, it isn't. I'm arguing that censorship inherently results in compromising privacy and security for all.