SSB is the first relatively successful social-networking protocol using public-keys that had any traction, it showed the way to Nostr.
- Because it is optimized for "local" social networks -- i.e. people that actually meet each other, SSB has some design choices around the structuring of each person's feed (each note must reference the previous one in a single chain of events) to make it easy to sync. These choices are actually very limiting in practice and contributed for SSB to not gain the adoption it deserved.
- For the same reason above, SSB has a the UX problem of not allowing people to hold the same identity in multiple devices and apps, which drastically limits its scope and scale.
- Moreover, since sharing feeds on the internet is an afterthought for SSB, it is really hard to get started and browse others' feeds, and thus to gain traction outside of well-established closed groups of interest.
- The protocol is unnecessarily complex.