But how do you do that without essentially downloading the whole social network to your local machine? Are other people's comments, quotes, likes, moderation signals something that should stay on the server or should be synced to the client for offline use? In the first case, you can't really use the social network without connecting to a server. The second case is a privacy and resources nightmare (privacy, because you can hold posts and comments from users that have deleted their data or banned you, you can see who follows who etc. Resources, because you need to hold the whole social graph in your local client).
Usually folks looking for this sort of social network are also looking for a more intimate social experience, so we're not necessarily talking about sync'ing the whole Twitter feed firehose.
I don't think it's unreasonable from a resources perspective to sync the posts/actions of mutual followers, and from a privacy standpoint it's not really any worse than your friend screenshotting a text message from you.
Sure, but they're a tiny fraction of the mainstream users and you can already have that sort of experience with blogging and microblogging. Relevant social networks as the public knows them are hard to develop local-first. Even the humble forum where strangers meet to discuss is really hard to do that way. If it needs centralized moderation, or a relevance system via karma / votes, it's hard.
(unless you want another paradigm of social networking in which you don't have likes, public follows, replies etc., which won't probably fly because it has a much worse UX compared to established social networks)
