Too true. It's getting tiring reading the comments of posts like this because I know it's going to be filled with people boasting about how they've already deleted their accounts/just did it. The HN crowd is a tiny, tiny minority in the pool of Facebook users, our actions are hardly representative of sea change.
Because people want to keep in touch with their friends, "friends", followers, etc.. They need to notify them that they are using some other way to communicate.