This has happened to multiple people I know who have deleted a Facebook account. Shortly after, a fake account with their picture and info popped up. I wonder if there are bots scanning for this waiting for people to delete accounts.
I have a facebook extension that alerts when a friend vanishes from my friend list. It does a lot of other stuff too, like remove ads etc. But that's one of its features and it's super easy to detect apparently. Facebook doesn't notify you for obvious reasons, but if you simply maintain a list of friends between visits it's easy.