The article says the girl is suing both the offending individual and Facebook. There's nothing about a law preventing the individual from being held responsible. If there is a relevant law as described where is it?
According to the article Facebook has the ability to control reposting of any material deemed unacceptable. I'd guess they could also block the accounts of offending individuals who violate policy standards or ignore them repeatedly. Facebook seems to have failed to control the situation where they could have.
The episode with the very famous photo of the "Napalm Girl" points to the usual problem. How to identify inappropriate material. The Vietnam photo shows a girl who survived being firebombed and had to strip her clothes off to avoid burning to death. A photojournalist documented the event showing a nude young girl running naked and screaming. Is it to be banned because of her age or is it a newsworthy terrifying incident? As it says Facebook eventually allowed it. If Facebook had a policy of no personal nudity at all, which they don't seem to, it would or should simplify the issue.
The girl's lawyer in this case said the posting of the nude photo was an act of revenge. The issue of revenge may be relevant in the suit but it should not be to Facebook. These days there is plenty of technology in widespread use to recognize a problematic issue of this kind and resolve it fast. Given that Facebook should be liable for failing to use tech applications they had already to stop this.
It must be difficult for Facebook or any other media to be able to keep track and be able to consistently block this kind of smut.
Smut? Now there's a definition many would have wide-ranging views on depending on personal tastes, along with the fact nudity or a sexual act is not absolutely necessary to be pornographic subjects. These things are very tricky.