I feel that if it is a personal post, then the carrier shouldn't be liable for it. So if someone is bullying Little Timmy, then Facebook shouldn't be the one on the hook for that (Sorry Mr. Hunt, I don't agree with your soundbite from the other day). I wouldn't blame Royal Mail for a nasty card I got from somebody.
However, advertisers are paying for the carrier to target people, so carriers should certainly be liable for what they are choosing to allow to be sent. In the Royal Mail example, if they send me some marketing bumf which is particularly awful and if a massive scam (and have gotten paid to deliver it) I think they should be on the hook for that. The same goes for Facebook. Honestly their excuse seems to be more "we don't vet stuff, we just let them pay to target people".