You could argue e-mail is to some degree decentralized, anybody can run their own server and send and receive email. I'm sure people right now are using it for

You could argue e-mail is to some degree decentralized, anybody can run their own server and send and receive email. I'm sure people right now are using it for hate speech and illegal porn. If someone sends you via email dubious content, you are still allowed to block it and report it. If we follow your logic we would have 20 different companies with their email variations just like we have today with chat messages. In the case of social networks, a feature could be aggregation and sharing of ban lists (i.e. I have my ban list, other people that trust me can use mine and merge with theirs, servers can host 'centralized' ban lists that people are free to add or ignore, kinda like dns based ad-blocking).