I feel like the most useful feature a distributed social network needs is the ability to grant/deny access to people based on decentralised identifiers (such as the person's email addresses, although people may prefer to use a dedicated email address for this purpose).
If such a distributed ID system were available, then it should even be possible to move away from one web host to another host, and still have access to your friends' pages, and have them find you. Alternatively you could instruct your current host to broadcast an "I'm about to move" message to your friends before you move your data to the new host.
Perhaps the biggest hurdle standing in the way of this is the spam / privacy issue, since people don't want to have to prove they own a specific email address in order to read your posts on the niche site where you're hosting your profile. Also, if your posts are being pushed out to the sites where your friends have accounts, then you have to implicitly trust the privacy policies and security of all those sites.