I think that's a very hard one for social networks, which are designed to solicit and then keep information. That's the way they run. I don't believe they've been designed from the ground up to easily delete information permanently and to guarantee, on an auditable basis, that it's been done, whereas a hospital or something else that has a more robust data-handling information services background might be able to do that.
The problem they often cite is that they make backups. Then the backup has it. And then a previous backup has it, and you'll never get rid of it. So, theoretically, we can always get to it.
But it's just a system design thing. If it's required, then I'm sure they'll be able to design it into the next version.