If the distributed social networks ever get enough users to attract spammers, they'll have an even worse spam problem than the centralized ones. They don't collect enough info network-wide to detect spammers.
I feel like that particular issue could be resolved with even the most rudimentary reputation system. It’s how humans have solved the problem amongst ourselves. But I agree it will be a problem.
I’m not trying to be negative. I’m just strongly of the opinion that making this happen will take much more than saying “we should make a better social media”, and I’m wondering if you’ve got that far.