It is not really deleted, it is deactivated...you can reactivate if you log back on...maybe there is a time limit, say 3 years until they actually delete inactive FB accounts.
Actually, there is a way to permanently delete it. It is several steps and you cannot log in for two weeks.
But it IS permanent. Not easy though.
They make it far too difficult.
Found a link that does allow you to delete...after 14 days:
https://www.facebook.com/help/contact.php?show_form=delete_account