Yes, lots of people have at least some of their photos only stored on FB and have no backups, so FB ends up being their de facto "photo backup service".

Obviously you should routinely backup your data, yet here we are. Last year I noticed that most of the photo albums I shared on FB over the years just disappeared. They were not hidden by some app setting, nor temporarily unavailable, the data dump FB offers has no trace they even ever existed. The data was gone. I reported the issue but never got any answer. Thankfully I had the original photos on my old desktop. So when your data may randomly vanish, do backups, just my two cents.

It isn't a great backup service, but it is useful. Every day they give me a x years ago you shared - normally a picture of my kids doing something cute that I forgot about until then. Brings a big smile to my face.

You have to be careful how you use Facebook. I've gotten aggressive about blocking all from every single group and cute kitten post. Facebook is a great way to share pictures with my friends and family. For news, jokes, buying/selling, and learning about my hobbies it is useless (because the algorithm doesn't show me everything). As a rules, if it isn't something you wouldn't mark as personal information with limited distribution it shouldn't be on Facebook. (this also implies Facebook needs high security)