[Joeboy]

Not sure I understand. If Facebook were federated, it would not be reasonable to expect it to delete your data for the practical reasons you mention. Facebook is not federated, so they presumably are able to delete your data. Maybe it's naive to expect them to behave in an honest or considerate manner, but not it's not unreasonable to ask it as far as I can see.

Edit: Of course there's nothing they can do if somebody's screenshotted your post or stored its contents in their brain, but that's not what we're talking about.