Decentralized Social Media: What is it, how does it work?

In ActivityPub you get a bit more resilience in that other people's instances might go down, but once they're up again you'll resume synchronizing with them. Your main issue is that once your instance goes down, you personally can't participate anymore unless you make an account somewhere else.

AT protocol is a bit more complicated in that you have several different points of failure. If the firehose goes down none of the app views will see new posts but should have their existing ones. If an app view goes down others will still work and you'd still be able to pull from people's PDSs. If your PDS goes down you can't post but if someone else's goes down you can still see everything else.h

Nostr has the most resilient model in that you can use as many relays as you want and if some of them go down you'd be fine so long as you can find more.