I’m not familiar with the sextortion case, but I do have a general answer.

Firstly, unlike the phone analogy, social media is not just one-to-one communication. It is one-to-many.

If a news outlet published hate speech or bullying or some other criminal activity in its paper or on TV, that outlet would also have repercussions as well as the individual. Just like if I were given an official spot in a town square and used it to shout libel all day, the town then bears some of the responsibility for giving me that platform. The same holds true of social media. 

Secondly, working in a digital space where the social channel itself controls much of where and how public content is shown (eg. the algorithm) they do hold specific responsibility for what a person might see.

This is why their capability is important. Facebook can’t necessarily stop one person starting to privately bully another. But they can allow users to be blocked,  stopping it from continuing. They can provide data to authorities if it turns into a criminal case. They can support identifying the material shared over their platform. If they were able to know that a pedo ring were operating and swapping pictures, but did nothing about it, they would be held partially liable for supporting it.