What if they accidentally deleted everything and don't have a backup?
If you mean all code it will be the end of Facebook.
But ofcourse this is never going to happen.
Once you delete something it stays on facebook for like weeks.
They would have to have accidentally blown up everything with a bomb to have the problem you're talking about.
As for not deleting data that you request to be deleted, that's normal practice. I know on every site I run, when someone clicks delete, it flags the content and it's hidden, but remains in the database. Deleted data is still valuable data, and it has a wide variety of uses.