Damaging in different ways. I'm not sure you can say one is worse.

Sure most kids can look at naked people and not be too affected, we all have the same parts. But beyond that, a lot of really harmful behavior is depicted in some porn and kids are not really mature enough to understand that in real healthy relationships people don't actually have sex like that.

Both porn and social media can be addictive and unhealthy if they become a substitute for interacting with real people. And this also happens with adults not just kids.

Facebook is over 20 years old and has never enforced its minimum age (13 years.)

I, for one, would have started with legal threats and financial penalties long ago. But it just won't happen. So I'm fine with technical solutions.

Facebook is able to sell you ads based on your favorite shoe lace colour. They ban terrorists, bots, porn and people named "Mark Zuckerberg" all the time. Noone can claim it's too hard for them to ban minors under 13.

Porn is a special subset of social media.