It's not practical to perform a delete in some environments. It's certainly not practical to go through backup files and delete references in others. Everybody goes through a data recovery scenario at some point.
What you should expect is best effort. Nobody is going to re-train their neural networks that referenced your data. It's doubtful that any provider would delete what they have gleaned about you simply because you wanted your account removed.
Then there's the practicality of things. If you delete your account here on HN, would you expect all conversations that you participated in to be removed? All references to your comments? Or all comments that you made removed? Do you expect them to push that change out to google? Do you expect your references in access logs removed? There's always a trail.
Facebook isn't uniquely evil. They have the same problems as everyone else, and they have those problems at an incredible scale.
People think about these things as if they are files or entries in a simple SQL database that can simply be removed. They may use databases, but to think that all of your data exists in a singular database or that all of your data only exists in databases is simply not extrapolating what you know about technology to their environment.
If google were to remove a website from their index, how long would it take for all of their environments to no longer have a record of it? And would that actually be desirable? There aren't faceless drones working on these kinds of problems. There are very smart people and most of them have a conscience and care about end users. I don't think you would find that any large group of people at facebook, google, amazon, microsoft, or any of the big companies are completely apathetic to end user concerns. I'm sure plenty are far too busy to address individuals directly, but they certainly are not apathetic.
<sorry... I seem to have rambled off on a tangent>