It means deleting your Facebook account will also prevent you logging into those other services. I had to delete a Spotify account and start over after I initially signed up via a Facebook login.

(I should note that the Spotify staff were amazingly helpful and actually migrated me over to an email-login account when asked.)

That's because you used FB to register instead of creating your own account. I suggest doing that in future, and use a password manager.

At one point Spotify were requiring users to link their accounts to Facebook. http://www.whathifi.com/news/spotify-no-longer-requires-face...

Ouch. Big reason not to register.

SyneRyder's post (https://news.ycombinator.com/item?id=11569891) was specifically adduced to demonstrate the dangers of logging in with Facebook, and mentions that doing so was an accident, so explaining the dangers and suggesting that he or she not do it again seems patronising.