This happened to my mom too. She told me she can't do any more photos because her storage is full. So I checked her android phone a and somehow the cloud backup got activated (we explicitly turned that off!!) and our family photos were uploaded to google cloud..
discoverability, spam and community moderation are already big problems in slightly less centralized systems - they're even more unsolved with these. as with activitypub, things work fine until suddenly there's enough adoption that you have an entire different class of problem.
https://nostr.com/