1. The whole Cambridge Analytica issue was caused by APIs that are too open. For distributed systems there are more ways to exploit the APIs and gather data on users.
2. There is a clear issue with Facebook's accountability in these areas. Distributed systems are typically open source, they run on multiple servers by different owners, this leads to zero accountability.
3. GDPR compliance about deleting data is almost impossible in a distributed system.
4. Some of the problems with Facebook are more about usability and clarifying how things work to users. For instance the scandal with people giving away access to their private messages. Open source software and distributed software tends to be much harder to use.
5. Any future concern/issue will be much harder to resolve if there are thousands of different instance running decentralized social networks.
6. Using AI to detect abusive content or spot fake news is much harder if you only have a subset of the data. So it becomes harder to address those concerns in a distributed setting.
So while I think this stuff is awesome from a tech perspective, in many ways it just makes these problems harder to solve.