If the answer is predetermined and known it isn't a question. It is some sort of formal whipping and there is no point giving any answer.
From the peanut gallery it is plausible that Facebook is a net win or a net loss for adolescents and it is possible that the company has a staff psychologist who has actually thought about its impacts. That question is so hard to answer in the negative I seem to have 2 people answering in this comment chain as though the frame is obviously correct and Zuckerberg needs to answer yes then atone for his sins. Public hearings are not the place to engage with these sorts of questions (there might be consequences if he makes a mistake in his answers).
The right answer to "have you stopped beating your wife?" is not "yes" if you don't actually beat your wife.
Similarly, the answer to "did you know the thing you were doing was evil?" is not "no" if the thing you did was not evil. Answering like that is conceding the point.
But they have conceded the point. I think that's why things like Cambridge Analytica were mentioned so many times during the hearing.
Anything you say can and will...
The right answer to this question is to accept that the company is extracting / harming people. And admit that they didn't stop to think about that when they were building it. That's understandable - they had no idea the impact that facebook would have on society when they were building it.
You can then go on to have a productive collaborative discussion about what to do about it (which isn't obvious or easy). The environment is hostile because Zuckerberg isn't engaging in the process, and is stonewalling congress with "we have a process for this".
He's not being honest, and I suspect (and hope tbh) it will eventually be the downfall of his company.