This HN meme that all companies should be like Apple is getting a bit ridiculous. FB has always been an efficient engineering organizing that pushes out features constantly. They don't derive their value from their UI nor have they ever. There is no reason for them to start copying Apple now.
FB users don't use it because of the slick interfaces, they use it because of network effects. For that type of business, adding features fast (to create more lock-in) > focusing on great UI.