Facebook is a complex piece of software engineered to deliver social data quickly.
Almost every piece of data on Facebook is socially linked. For example,
If you delete a message to a friend, the message will remain in their inbox.
Maintaining these data relationships makes deletion difficult. Even data tagged for deletion can still take 90 days to be deleted.
‘It typically takes about one month to delete an account, but some information may remain in backup copies and logs for up to 90 days.’ — Facebook Data Use Policy.
Keep in mind that Facebook's value involves retaining as much social data as possible. Instant data removal is not the priority.
‘For content that is covered by intellectual property rights, like photos and videos (IP content), you specifically give us the following permission, subject to your privacy and application settings: you grant us a non-exclusive, transferable, sub-licensable, royalty-free, worldwide license to use any IP content that you post on or in connection with Facebook (IP License). This IP License ends when you delete your IP content or your account unless your content has been shared with others, and they have not deleted it.’ — Facebook's Statement of Rights and Responsibilities.