I harbor serious doubts that most of the 'volunteers' here know exactly what it is they're providing -- the sign-up sheet probably didn't say "we will know very specifically your porn-watching habits" e.g.
I think that this is a fairly common when it comes to technology. The terms and conditions seem reasonable ("we collect some data to provide more relevant ads"), but when you look a bit more closely they build a personal file that contains who you communicate (email/text/call) with and how often, where you go, what you buy, which websites you visit, which videos you watch, etc to the extent that they are able. My mother is very smart, well-educated (she has a PhD), and relatively tech savvy (she works in scientific computing), but she was still floored when I told her about some of the tracking Facebook and Google perform. Google recording her location (which she technically agreed to, but did not realize) was enough that she asked me to help her migrate away from Gmail. She probably would have managed without my assistance, but the barrier would have been much higher.
"We will hold logs of you saying awful things to your girlfriend as you're breaking up in a file on you for the next 50 years" is more accurate. Privacy nihilism comes either from a lack of imagination, or a lack of perceived power.
13 to 17 year olds aren't supposed to be able to access porn legally, so can Facebook plausibly deny that this is something they are monitoring?
FB is not supposed to make deals with minors without adult supervision…
It was open to adults as well, but yes. However, they had no duty to specifically enumerate that particular case anyway lol. And technically that is a crime under US law to distribute porn to minors, but its not a crime for the users to view it, so if they connect to a website intended for and operating in another country without such laws, there is no legal issue.