I manage and have set up various Facebook groups related to my musical interests - a couple of folk clubs and two ceilidh bands and am an “admin” on a couple of others. I suppose one of my golden rules is that any photos that go on these Facebook group pages are also saved to my hard disk and backed up as well: I certainly would not rely on Facebook to keep them indefinitely. Many of them are mine, but I also save photos that other people post: admittedly they may be of low res, but if need be, I can usually ask the photographer for a higher res original if that seems necessary.
In setting up new groups, I also did a big back trawl through emails, looking for photos that might have previously been sent around the group as email attachments. In my email system (Yahoo) you can ask for a search of attachments rather than every single email you’ve not deleted. This retrieved quite a few, and I duly saved them as above. Took a bit of time and effort, but hopefully worth it.