The claim of catching 99% of activity is really hard to justify. In many detection systems you cannot know your true number of false negatives. It's inherently unknowable in this case. A better metric would be mean time to detection for a given CEP sharer or perhaps a metric around how much CEP is shared before detection, or perhaps a metric for network size of CEP and how well FB can clean up an entire network at on…
I'll try my hand here:
You're talking about this as "preventing child abuse" versus "privacy". The former is pretty concrete, while the latter is pretty abstract. But allowing government to invade everyone's privacy has concrete effects. Let's be clear here: in 2013, 2.2 million people were incarcerated in the US.[1] 1 in 5 is locked up for a nonviolent drug offense[2]--that's 440,000 people. And 540,000 people are incarcerated because they can't afford bail. It's unclear from the graphs on that page how many are locked up for child abuse, but the total incarcerated for rape and sexual assault convictions is 163,000 people in state jails, which by all accounts is where most rape and sexual assault convicts end up. It's unclear how many of those rape and sexual assault cases were convicted due to evidence from surveillance, but I think we can assume that it wasn't all of them. And in case it's not clear, quite a few of those unethically incarcerated will be raped[3]. In short, unethical incarceration is a much larger threat, by the numbers, than child abuse.
We've seen time and time again that violations of privacy are publicized as being used to prevent terrorism and child abuse, but immediately are then used to prosecute nonviolent drug offenders.
What it comes down to for me is that I trust average citizens to do the right thing more than I trust the law. The vanguard of law enforcement is average citizens picking up the phone and calling the police when they see something wrong, and I want to keep it that way. I trust the average person to call the police when they witness a crime like murder, rape, or child abuse, and not call the police when they witness drug possession. I do not trust law enforcement to enforce the law as ethically. Pervasive surveillance takes that power out of the hands of citizens and puts it in the hands of law enforcement who has time and time again shown themselves to be untrustworthy with that power. It's dangerous to be right when the government is wrong.
And that's in the current US. In the past, US law has been even more wrong. For example, US law enforcement was used to suppress, for example, civil rights activists. That time may come again. It has always been beneficial to let illegal behavior slide under the gaze of law enforcement when that illegal behavior wasn't unethical behavior.
[1] https://en.wikipedia.org/wiki/Incarceration_in_the_United_St...
[2] https://www.prisonpolicy.org/reports/pie2019.html
[3] https://en.wikipedia.org/wiki/Prison_rape_in_the_United_Stat...