I agree, the question remains useful. It's now a question about the history of security mechanisms instead of current usage of security mechanisms. It's also a question about when to use and not to use HSTS, just as it was before.
The question and, where relevant, should be updated to not imply that Facebook currently doesn't use HSTS. (Done, at least for the question.) It would be a good complement for answers to explain why Facebook changed their policy, as it would inform the decision of someone deciding whether to use HSTS for their site.
There's no reason to close the question. It hasn't magically stopped being on-topic or answerable. There's still room for someone to post a better answer. The reason we close questions is because we don't want them to be answered, for one reason or another (already answered elsewhere, not the right place for people to evaluate answers, impossible to write a good answer, …). Answers to this question still remain worthwhile an on-topic, so the question should remain open.