I would completely agree that people are bad at interpreting Down Detector-type results, but that doesn't mean it isn't providing a very useful signal.
Indeed I haven't noticed any blip in functionality, but then again I don't ever do FB (or other external service) login. Absolutely no reason to do so, long term drawbacks are too serious to be lazy about this.
Scroll up the thread a bit; https://news.ycombinator.com/item?id=39605354
Even here on HN, where people should know better, people take its incorrect attribution as useful info. TikTok isn't down. X isn't down. Google isn't down.