I would love to see Facebook, Twitter, and (much harder but maybe even) YouTube replaced by distributed systems, so they would be more like email, RSS, or the web. You would have many different options for hosts, including self-hosting, and you would subscribe to the feeds you cared about that would be passed around via an open protocol.

You would get the "addresses" of other participants the way you get their email addresses today, and various directories would probably spring up to allow you to put your address on a list of people willing to be found. Your own apps would format your content for the feed and draw from the various feeds to construct your "timelines".

I think that for Facebook and Twitter, the value due to network effects is far more important than the benefits that result from centralization. Everyone wants to go where everyone else is more than they want whatever specific features might require centralization.