“He who controls the past controls the future. He who controls the present controls the past.”
― George Orwell, 1984
When an Austrian privacy activist requested to see his personal data that Facebook stored on its servers, he found the information included phone numbers and email addresses of his friends and family; a history of all the devices he used to log in to the service; all the events he had been invited to; everyone he had “friended” (and subsequently de-friended); and an archive of his private messages.
It even included transcripts of messages he’d deleted. Full story (BBC)