This raises several interesting questions.
Without doubt the person who put up the images should be held to account and why Facebook can't/won't identify them is a very good question.
However, are Facebook liable as well. Some people are citing carrier exemption, but we'er talking about two different scenarios. With the Royal Mail and ISPs, they are simply transporting the goods and do not 'host' them. This is somewhat different to Facebook, where they are actually hosting and providing services for the images to be seen. To my mind, this is entirely different. After all, you could equate Facebook to a magazine. In both cases, they're providing the material and hosting it, not just transporting it. Now, if someone sold a magazine with a picture (presumably compromising) of a minor, would they not be prosecuted? What's the difference? If someone sent a photo into a magazine and they published it, what's the difference?
It rather strikes me that Facebook are guilty of breaching several laws in this country, just as much as any other site hosting material such as pictures. After all, if you sent material into a magazine, they simply automated into the magazine and then distributed or sold it, would you not expect them to check the content? What's the difference? Obscene publications, child abuse etc.? Would we accept excuses such as this from a magazine producer?