The most offensive to modern sensibilities (and advertisers) isn't the naked people, but the gore, self harm, and extreme fetish communities.
Ok, Facebook presumably dodges the problem with privacy settings. But it's no harder to post an underage selfie on Reddit or Twitter than on Tumblr, and those sites have much larger userbases. Their apps haven't been hit by Apple, nor have they ever talked about resorting to outright adult-content bans to control it.
So what's the difference? Are there any major theories other than "tumblr's site moderation tools sucked" and "tumblr's moderation team was way too small"?
On the other hand, there's just as much harm from the no-shame communities - look up pro-ana for an example, and the realize that groups like this exist for many forms of self harm, including cutting, drug abuse, and (and this one is personal to me) rape.
https://youtu.be/Y7lYeRqhQ9Q is a good example of how young people can be drawn into these sorts of toxic relationships. And he worst part is: most of the warning signs aren't a problem in and of themselves. It is only in patterns and degrees that you realize that there's something deeply wrong, and the behavior has crossed a line. The lines aren't bright and clear - but, to some extent, the people who wander even near and don't immediately turn away are the ones you know are willing to cross it.