For most non-HN types I think the answer is FB and Instagram. but the posting has trended away from the actual 'family memories' photos many of us would want to save. Wouldn't want to bet on that being there 15 years from now though.
Facebook images have such low quality, and posts tend to randomly disappear. I wouldn't use it as any kind of archiving tool.
Yeah for people that actually know about that or care. But for like my mom, that's where she posts and stores stuff. All the grandkid photos that's the only place they exist off her phone which probably does not have great backups